LYCOPODIALES.

The recent members of the Lycopodiales are considered apart from the extinct genera in order that our examination of the latter may be facilitated by a knowledge of the salient characteristics of the surviving types of this important section of the Pteridophyta. A general acquaintance with the extinct as well as with the recent genera will enable us to appreciate the contrasts between the living and the fossil forms and to realise the prominent position occupied by this group in the Palaeozoic period, a position in striking contrast to the part played by the diminutive survivors in the vegetation of the present day. In the account of the recent genera special attention is drawn to such features as afford a clue to the interpretation of the fossils, and the point of view adopted, which at times may appear to lead to an excessive attention to details, is necessarily somewhat different from that represented in botanical text-books[83].

A. HOMOSPOREAE.

Lycopodiaceae: genera Phylloglossum, Lycopodium.

B. HETEROSPOREAE.

Selaginellaceae: genus Selaginella.
Isoetaceae: genus Isoetes.

The existing plants included in the Lycopodiales are in nearly all cases perennial herbaceous pteridophytes, exhibiting in their life-histories a well marked alternation of generations. The sporophyte (asexual generation) is characterised by the relatively small size of the leaves except in the genus Isoetes ([fig. 132]) and in the Australian and New Zealand genus Phylloglossum. The stems are usually erect or trailing, pendulous in epiphytic species or small and tuberous in Isoetes and Phylloglossum. The repeated forking of the shoots (monopodial and dichotomous branching) is a prominent feature of the group. The vascular tissue of the stem usually assumes the form of a single axial strand (stele) ([fig. 125]), but the shoots of some species of Selaginella often contain two or more distinct steles ([fig. 131]). The group as a whole is characterised by the centripetal development of the xylem composed almost entirely of scalariform tracheids: secondary xylem and phloem of a peculiar type occur in Isoetes, and the production of secondary xylem elements in a very slight degree has been noticed in one species of Selaginella (S. spinosa)[84]. The roots are constructed on a simple plan, having in most cases only one strand of spiral protoxylem elements (monarch structure). In Lycopodium, in which stem and root anatomy are more nearly of the same type than in the majority of plants, several protoxylem strands may be present. The sporangia are axillary or, more frequently, borne on the upper surface of sporophylls, which are either identical with or more or less distinct from the foliage leaves; in the latter case the sporophylls often occur in the form of a well defined strobilus (cone) at the tips of branches.

The gametophyte (sexual generation) is represented by prothalli which, in the homosporous genera, may live underground as saprophytes, or the upper portion may develop chlorophyll and project above the surface of the ground as an irregularly lobed green structure (e.g. Lycopodium cernuum)[85]. In the heterosporous forms the prothalli are much reduced and do not lead an independent existence outside the spore by the membrane of which they are always more or less enclosed. The sexual organs are represented by antheridia and archegonia; the male cells are provided with two cilia except in Isoetes which has multiciliate antherozoids like those of the ferns.

The existing Lycopods, though widely distributed, never grow in sufficiently dense masses to the exclusion of other plants to form a conspicuous feature in the vegetation of a country. The inconspicuous rôle which they play among the plant-associations of the present era affords a striking contrast to the abundance of the arborescent species in the Palaeozoic forests of the northern hemisphere.

•••••

Lycopodiaceae. Lycopodium, represented by nearly 100 species, forms a constituent of most floras: epiphytic species predominate in tropical regions, while others flourish on the mountains and moorlands of Britain and in other extra-tropical countries. For the most part Lycopodium exhibits a preference for a moist climate and appears to be well adapted to habitats where the amount of sunlight is relatively small and the conditions of life unfavourable for dense vegetation. Mountains and islands constantly recur as situations from which species have been recorded. Some species are essentially swamp-plants, e.g. Lycopodium inundatum, a British species, and L. cruentum from the marshes of Sierra Nevada. A variety of the American species, L. alopecuroides (var. aquaticum) affords an instance of a submerged form, which has been collected from an altitude of 12–14,000 ft. on the Andes and Himalayas. It is noteworthy that a considerable variety of habitats is represented within the limits of the genus and that many species are sufficiently hardy to exist in circumstances which would be intolerable to the majority of flowering plants[86].

The British species frequently spoken of as Club Mosses, include Lycopodium Selago, L. annotinum, L. clavatum, L. alpinum, and L. inundatum.

•••••

Selaginellaceae. The species of Selaginella, over 300 in number, are widely spread in tropical and subtropical forests, growing on the ground with trailing, suberect or erect stems climbing over taller and stouter plants or as pendulous epiphytes on forest trees.

Selaginella lepidophylla, a tropical American type, popularly known as the Resurrection plant, and often erroneously spoken of as the Rose of Jericho[87], possesses the power of rolling up its shoots during periods of drought and furnishes an example of a species adapted to conditions in marked contrast to those which are most favourable to the majority of species.

The only British species is Selaginella spinosa named by Linnaeus Lycopodium selaginoides and occasionally referred to as Selaginella spinulosa A. Br. (not to be confounded with a Javan species S. spinulosa Spring[88]).

•••••

Isoetaceae. Isoetes ([fig. 132]), of which Mr Baker in his Handbook of the Fern-Allies enumerates 49 species, is a type apart, differing in habit as in certain other characters from the other members of the Lycopodiales. Some botanists[89] prefer to include the genus among the Filicales, but the balance of evidence, including resemblances between Isoetes and extinct Lycopodiaceous plants, would seem to favour its retention as an aberrant genus of the group Lycopodiales. Some species are permanently submerged, others occur in situations intermittently covered with water, and a few grow in damp soil. Isoetes lacustris is found in mountain tarns and lakes of Britain and elsewhere in Central and Northern Europe and North America. Isoetes hystrix[90], a land-form occurs in Guernsey, North-East France, Spain and Asia Minor.

Lycopodiaceae.

The monotypic genus Phylloglossum, represented by P. Drummondii of Australia and New Zealand, though interesting from the point of view of its probable claim to be considered the most primitive type of existing Lycopodiaceous plants, need not be dealt with in detail. A complete individual, which does not exceed 4 or 5 cm. in length, consists of a very small tubercle or protocorm bearing a rosette of slender subulate leaves and prolonged distally as a simple naked axis which overtops the foliage leaves and terminates in a compact cluster of small scale-like sporophylls, each subtending a single sporangium[91].

Lycopodium. It would be out of place in a volume devoted mainly to fossil plants to attempt a comprehensive account of the general morphology of recent species, and indeed our knowledge of the anatomical characters of the genus is still somewhat meagre. For purposes of comparison with extinct types, it is essential that some of the more important morphological features of existing species should be briefly considered. The additions made to our knowledge of the gameophyte[92] of European and tropical species during the last two decades have revealed a striking diversity in habit.

In several species, grouped round the widely distributed type Lycopodium Selago Linn., the comparatively short, erect or suberect, shoots form fairly compact tufts; the ordinary foliage-leaves function as sporophylls, and the sporangia are not localised on special portions of shoots. From this type, we pass to others in which the fertile leaves tend to be confined to the tips of branches, but hardly differ in form from the sterile. A further degree of specialisation is exhibited by species with well-defined cones composed of leaves (or bracts), the primary function of which is to bear sporangia and to afford a protective covering to the strobilus[93].

Lycopodium rufescens Hook. An Andian species with stout dichotomously branched erect stems bears on the younger shoots crowded leaves with their thick and broadly triangular laminae pointing upwards, but on the older and thick shoots the laminae are strongly reflexed ([fig. 121], A). The lower part of the specimen represented in [fig. 121], A, shows tangentially elongated scars and persistent leaf-bases or cushions left on the stem after the removal of the free portions of the leathery leaves, a surface-feature which also characterises the Palaeozoic genus Lepidodendron. The reflexed leaves and persistent leaf-cushions are clearly seen in the piece of old stem of Lycopodium dichotomum Jacq., a tropical American species reproduced in [fig. 121], B. Such species as L. erythraeum Spring, and others with stiff lanceolate leaves exhibit a striking resemblance to the more slender shoots of some recent conifers, more especially Araucaria excelsa, A. Balansae, Cryptomeria, Dacrydium and other genera.

Fig. 121. Lycopodium.

  1. Lycopodium rufescens.
  2. L. dichotomum.
  3. L. tetragonum.
  4. L. nummularifolium.
  5. L. Dalhousianum.
  6. L. casuarinoides.
  7. L. volubile.

Fig. 122. Lycopodium squarrosum. The branches of the larger shoot terminate in cones. (From a plant in the Cambridge Botanic Garden. Reduced.)

In Lycopodium tetragonum Hook., ([fig. 121], C), a species from the Alpine region of the Andes, the long, pendulous and repeatedly forked branches bear four rows of fleshy ovate leaves and simulate the vegetative characters of certain conifers.

Fig. 123. Lycopodium cernuum.
(From a specimen in the Cambridge Herbarium. ½ nat. size.)

L. squarrosum Forst. ([fig. 122]) a tropical species from India, Polynesia, and other regions, is characterised by its stout stems reaching a diameter of 2·5 cm., bearing long pendulous branches with large terminal cones composed of sporophylls differing but slightly from the foliage leaves. The plant represented in the photograph serves as a good illustration of the practical identity in habit between Palaeozoic and recent genera.

Fig. 124. Lycopodium obscurum.

L. Dalhousianum Spring, from the mountains of the Malay Peninsula and Borneo, has larger leaves of finer texture with a distinct midrib reaching a length of 2–3 cm. ([fig. 121], E). Another type is illustrated by L. nummularifolium Blume, also a Malayan species, in which the leaves are shorter, broadly oblong or suborbicular, and the branches terminate in narrow and often very long strobili (sometimes reaching a length of 30 cm.) with small bracts in striking contrast to the foliage leaves ([fig. 121], D). A similar form of long and slender strobilus occurs in L. Phlegmaria Linn., a common tropical Lycopod: the frequent forking of the strobili noticed in this and other species is a character not unknown among fossil cones (Lepidostrobi).

L. cernuum Linn. ([fig. 123]), another widely spread tropical type, offers an even closer resemblance than L. squarrosum to the fossil Lepidodendra. The stiff erect stem, reaching in some cases a length of several feet, bears numerous repeatedly forked branches, with crowded linear leaves, terminating in short cylindrical cones with broadly ovate sporophylls. A similar habit characterises the North American species L. obscurum Linn. ([fig. 124]) bearing cones several centimetres in length.

L. casuarinoides Spring ([fig. 121], F) an eastern tropical species, is worthy of notice as exhibiting a peculiar form of leaf consisting of a very small lamina, 3 mm. in length, borne on the top of a long decurrent base, which forms a narrow type of leaf-cushion, bearing some resemblance to the long and rib-like cushions of certain species of Sigillaria, and recalling the habit of slender fossil twigs referred to the Coniferae under such names as Widdringtonites, Cyparissidium, Sphenolepidium.

L. volubile Forst. ([fig. 121], G) a New Zealand species, in habit and leaf-form bears a close resemblance to the Jurassic Lycopodites falcatus Lind. and Hutt. ([fig. 137]): it is also a representative of a few species of Lycopodium which agree with the majority of species of Selaginella in having two kinds of sterile leaves, comparatively long falcate leaves forming two lateral rows and smaller appressed leaves on the upper surface of the branches.

These examples suffice to illustrate the general appearance presented by the vegetative shoots of recent species of which the foliage leaves vary considerably—from the small scale-leaves of Lycopodium tetragonum, to the very slender linear subulate leaves of such a species as L. verticillatum Linn. or the long and broader lamina of L. Dalhousianum ([fig. 121], E). It is obvious that fragments of the various types preserved as fossils might well be mistaken either for some of the larger mosses or for twigs of conifers. As Dr Bommer[94] has pointed out in his interesting paper on “Les causes d’erreur dans l’étude des empreintes végétales” some dicotyledonous plants may also simulate the habit of Lycopods: he cites Phyllachne clavigera Hook (Candolleaceae), Tafalla graveolens Wedd (Compositae) and Lavoisiera lycopodioides Gard. (Melastomataceae). Another point illustrated by [fig. 121] is the close agreement in habit and in the form of the leaves and leaf-cushions between the recent plants and the Palaeozoic Lepidodendreae.

In his masterly essay “On the vegetation of the Carboniferous Period, as compared with that of the present day” Sir Joseph Hooker called attention to the variation in the shape and arrangement of the leaves in the same species of Lycopodium. The three woodcuts which he publishes of Lycopodium densum, a New Zealand species, afford striking examples of the diversity in habit and leaf-form and justify his warning “that if the species of Lepidodendron were as prone to vary in the foliage as are those of Lycopodium, our available means for distinguishing them are wholly insufficient[95].”

As we have already noticed, there is a considerable diversity among recent species, both as regards habitat and habit; in the anatomy of the stem also corresponding variations occur within the limits of a well-defined generic type of stele. In species with creeping stems, such as L. clavatum[96], the stele exhibits an arrangement of vascular tissue characteristic of the plagiotropic forms. The xylem consists of more or less horizontal plates of scalariform tracheae, each surrounded by small-celled parenchyma, alternating with bands or groups of somewhat ill-defined phloem. The protoxylem and protophloem elements occupy an external position (exarch), pointing to a centripetal development of the metaxylem. This centripetal or root-like character of the primary xylem is an important feature in recent as in fossil Lycopods. The close agreement between the roots and stems of recent species in the disposition of the vascular elements also denotes a simpler type of anatomy than occurs in the majority of vascular plants in which stem and root have more pronounced structural peculiarities. A pericycle, 2–6 cells in breadth, encloses the xylem and phloem bands and this is succeeded by an endodermis, 2–3 cells broad, with vaguely defined limits. In L. clavatum, as in L. alpinum, another British species, the broad cortex is differentiated into three fairly distinct regions; abutting on the endodermis is a zone several layers broad of thick-walled cells constituting an inner cortex modified for protection and support; the central region consists of larger and thinner-walled cells adapted for water-storage and aeration; beyond this is an outer cortical zone of firmer and thicker elements. The prominent leaf-bases or leaf-cushions ([fig. 125], A, lc) give to the surface of a transverse section a characteristic appearance which presents the closest agreement with that of the younger shoots of Lepidodendron. From the peripheral protoxylem groups small strands of xylem are given off, which follow a steeply ascending course through the cortex to the single-veined leaves. The leaf-traces, in several species at least, are characterised by a mesarch structure ([fig. 125], F, G), the spiral protoxylem elements occupying an approximately central position. The mesophyll of the leaves varies in regard to the extent of differentiation into a palisade and spongy parenchyma; in all cases there is a single vascular bundle occasionally accompanied by a secretory duct.

Fig. 125.

  1. Lycopodium dichotomum. Transverse section of stem: lc, leaf-cushion; lt, leaf-trace; R, roots.
  2. L. cernuum, portion of cortex of fig. H, enlarged.
  3. L. saururus. Cortex: lt, leaf-trace; a, thin-walled tissue; b, thick-walled tissue; lc, lacuna.
  4. L. saururus. Stele: x, xylem; p, phloem.
  5. Portion of fig. D, enlarged: px, protoxylem; p, phloem.
  6. Transverse section of leaf of Lycopodium.
  7. Vascular bundle of leaf: px, protoxylem.
  8. L. cernuum: b, branch of stele; c–c″, cortex; s, space in cortex; lt, leaf-trace.
  9. Stele of fig. H, enlarged (phloem omitted).

In erect stems of Lycopodium, as represented by L. cernuum (figs. [123], [125], H, I), L. Dalhousianum, L. squarrosum ([fig. 122]) and many others, the stele presents a characteristic appearance due to the xylem plates being broken up into detached groups or short uniseriate bands with the interspaces occupied by phloem islands. This type of structure bears a superficial resemblance to that in the single stele of certain species of the fern Lygodium[97], but it is distinguished by the islands of phloem scattered through the stele. In other species the xylem tends to assume the form of a Maltese cross (e.g. L. serratum Thbg.) or it may be disposed as V-shaped and sinuous bands terminating in broad truncate ends composed of protoxylem elements. This form of the xylem and the distribution of the phloem groups are shown in [fig. 125], D, E, drawn from a section of a plant of Lycopodium saururus Lam.[98] collected by Mr A. W. Hill at an altitude of 15,000 feet on the Andes of Peru. The position of the protoxylem is shown [fig. 125], E, px.

While several species possess a cortex of three distinct zones ([fig. 125], H, c, c′, c″), in others the extra-stelar tissue is much more homogeneous, consisting of thin-walled parenchyma or in some cases of thick-walled elements; as a general rule, however, there is a tendency towards a more compact arrangement in the inner and outer portions of the cortex as contrasted with the larger and more loosely connected cells of the middle region. In certain types the middle cortex contains fairly large spaces, as in the swamp-species L. inundatum, which with L. alopecuroides exhibits another feature of some interest first described by Hegelmaier[99]. If a transverse section of the stem of L. inundatum be examined the leaf-traces are seen to be accompanied by a circular canal containing mucilage which extends into the lamina of the leaf. In a specimen of L. cernuum[100] obtained at a height of 2500 ft. by Professor Stanley Gardiner in the Fiji Islands, the leaf-traces ([fig. 125], B lt) were found to be accompanied for part of their course by a well-marked secretory space ([fig. 125], B, s). There is little doubt that the presence of these mucilage canals is directly connected with a certain type of habitat[101] and attention is called to them in view of a resemblance which they offer to a characteristic strand of tissue, known as the parichnos, which is associated with the leaf-traces of Lepidodendreae and Sigillarieae. In the section shown in [fig. 125], H, the xylem of the stele forms more continuous bands than is often the case in L. cernuum which has already been described as having its xylem in small detached groups. The presence of the smaller branch-stele ([fig. 125], H, b) affords an example of monopodial branching. The outer cortex of L. saururus ([fig. 125], C) exhibits a somewhat unusual feature in the distribution of the thicker-walled tissue (b) which encloses a patch of more delicate parenchyma (a) with large lacunae (lc) in the region of the leaf-bases, and presents the appearance of an irregular reticulum. This arrangement of the mechanical tissue in the outer cortex is comparable with that in stems of some species of Sigillaria.

In certain species of Lycopodium the roots[102], which arise endogenously from the axial vascular cylinder, instead of passing through the cortex of the stem by the shortest route, bend downwards and bore their way in a more or less vertical direction before emerging at or near the base of the aerial shoot. The transverse section of L. dichotomum represented in [fig. 125], A, shows several roots (R) in the cortex; they consist of a xylem strand of circular or crescentric form accompanied by phloem and enclosed by several layers of root-cortex. The roots of Lycopodium do not always present so simple a structure as those of L. dichotomum; the xylem may have an irregularly stellate form with as many as ten protoxylem groups.

Reproductive Shoots[103]. In Lycopodium Selago the foliage leaves serve also as sporophylls and, as Professor Bower[104] has pointed out, the branches exhibit to some extent a zonal alternation of sterile and fertile leaves; in other species, in which foliage leaves and sporophylls are practically identical, the sporangia occur sporadically on the ordinary leaves. In species with well-defined terminal cones the lower sporophylls may bear arrested sporangia and thus form transitional stages between sterile and fertile leaves, a feature which occurs also in the male and female flowers of many recent Araucarieae[105]. The sporangia[106] ([fig. 126], D, F) are usually reniform and compressed in a direction parallel to the surface of the cone-scales; they are developed from the upper surface and close to the base of the fertile leaf to which they are attached by a short and thick stalk (e.g. L. inundatum) or by a longer and more slender pedicel (L. Phlegmaria, [fig. 126], E). On maturity the sporangia open as two valves in the plane of compression and the line of dehiscence is determined in some species at least by the occurrence of smaller cells in the wall. In transverse sections of cones in which the sporangia are strongly saddle-shaped, the sporophylls may appear to bear two sporangia. This is well shown in the section of a cone of L. clavatum shown in [fig. 126], F. The sporangia a and b are cut through in an approximately median plane showing the irregular outline of the sterile pad (p) of tissue in the sporogenous cavity. Those at c and d have been traversed at a lower level and the two lobes of the saddle-shaped sporangia are cut below the attachment to the sporophyll. The distal laminae of the sporophylls, cut at different levels, are seen at the periphery of the cone.

Fig. 126.

  1. Lycopodium cernuum, longitudinal section of strobilus; a, band of lignified cells.
  2. L. cernuum. Cell from sporangium wall.
  3. L. cernuum. Sporophyll and sporangium; lt, vascular bundle.
  4. L. clavatum. Part of radial longitudinal section of strobilus; p, sterile tissue.
  5. L. Phlegmaria. Sporophyll and stalked sporangium.
  6. L. clavatum. Transverse section of strobilus; p, sterile pad.

In longitudinal radial section of some cones the sporangia appear to occupy an axillary position, but in others (e.g. L. clavatum) they are attached to the horizontal portion of the sporophyll almost midway between the axis of the cone and the upturned distal end of the sporophyll ([fig. 126], D). The wall of a sporangium frequently consists of 2–3 cell-layers and in some cases (e.g. L. dichotomum), it may reach a thickness of seven layers, resembling in this respect the more bulky sporangia of a certain type of Lepidodendroid cone. The sporogenous tissue is separated from the stalk of the sporangium by a mass of parenchymatous tissue which may project as a prominent pad ([fig. 126], D, F, p) into the interior of the sporogenous cavity. This basal tissue (the subarchesporial pad of Bower[107]) has been observed in L. clavatum to send up irregular processes of sterile cells among the developing spores, suggesting a comparison with the trabeculae which form a characteristic feature of the sporangia of Isoetes and with similar sterile strands noticed by Bower[108] in Lepidostrobus (cone of Lepidodendron).

Each sporophyll is supplied by a single vascular bundle which according to published statements never sends a branch to the sporangium base. The fertile tips of the foliage shoots of L. cernuum ([figs. 126], A–C) afford good examples of specialised cones. The surface of the cone is covered by the broadly triangular laminae of sporophylls ([fig. 126], C) which in their fimbriate margins resemble the Palaeozoic cone-scales described by Dr Kidston[109] as Lepidostrobus fimbriatus. The distal portions of the sporophylls are prolonged downwards ([fig. 126], A) to afford protection to the lower sporangia, their efficiency being increased by the lignified and thicker walls (A, a) of the cells in the lower portion of the laminar expansion. The cells of the sporangial wall are provided with strengthening bands which in surface-view ([fig. 126], B) present the appearance of prominent pegs. Since the appearance of Miss Sykes’s paper on the sporangium-bearing organs of the Lycopodiaceae, Dr Lang[110] has published a more complete account of the structure of the strobilus of Lycopodium cernuum in which he records certain features of special interest. The importance of these morphological characters is increased by their agreement, as shown by Lang, with those of the Palaeozoic cone Spencerites[111]. The sporophylls of a cone (12 mm. long by 3 mm. in diameter) of Lycopodium cernuum show an abrupt transition from the foliage leaves, but like these they occur in alternate whorls of five. A large sporangium is attached to the upper face of each sporophyll close to the base of the obliquely vertical distal lamina ([fig. 127]); each sporophyll, which is supplied with a single vascular bundle, has a large mucilage-cavity (m) in its lower region. “The mucilaginous change” in the sub-sporangial portion of a sporophyll “extends to the surface involving the epidermis, so that this portion of the sporophyll-base may be described as consisting of a mass of mucilage bounded below by a structureless membrane[112].” Dehiscence of the sporangia occurs at the middle of the distal face ([fig. 127], x). As seen in the radial section ([fig. 127], ma) the outer margin of the base of the sporophyll bears a short outgrowth. The leaf-bases of each whorl hang down between the sporangia of the alternating whorl below, and the base of each sporophyll is coherent with the margins of the two sporophylls of the next lower whorl between which it lies, the sporangia being thus closely packed and lying in a pocket “open only on the outer surface of the cone.” Fig. 128 represents a transverse section through a cone in the plane AA of [fig. 127]; this traverses the sporangia and their subtending bracts (b) of one whorl and the dependent bases of the sporophylls of the next higher whorl in the region of the mucilage-sacs (m), which are bounded at the periphery by the outer tissue of the sporophylls (a). A transverse section in the plane BB of [fig. 127] is shown in [fig. 129]: the pedicels and a part of each vascular strand are seen at b radiating from the axis of the cone; one sporophyll (sp, a) is cut through in the region of the pad of tracheal tissue that characterises the short sporangial stalks. The upper portions of the sporangia of the next lower whorl, which project upwards against the mucilaginous bases of the sporophylls above (cf. [fig. 127], BB) are shown at c and external to them, at a, the section has cut through the outer persistent portions of these sporophyll bases.

Fig. 127. Radial longitudinal section of the cone of Lycopodium cernuum. (After Lang.)

Fig. 128. Transverse section of the cone of Lycopodium cernuum, in its plane AA of [fig. 127]. (After Lang.)

Fig. 129. Transverse section of the cone of Lycopodium cernuum in the plane BB of [fig. 127]. (After Lang.)

As Lang points out, this highly complex structure is an expression of the complete protection afforded to the sporangia of a plant met with in exposed situations in the tropics; it is also of importance from a morphological standpoint as exhibiting an agreement with the extinct type of Lycopod cone represented by Spencerites.

Selaginellaceae.

Selaginella differs from Lycopodium in the production of two kinds of spores, megaspores and microspores, and, in the great majority of species, in the dimorphic character of the foliage leaves, which are usually arranged in four rows, the laminae of the upper rows being very much smaller than those of the lower ([fig. 130], 1–3). The smaller leaves are shown more clearly in [fig. 130], 1a. It is obvious from an examination of a Selaginella shoot, such as is shown in [fig. 130], that in fossil specimens it would often be almost impossible to recognise the existence of two kinds of leaves. Some species, e.g. Selaginella spinosa[113], the sole British representative of the genus, are homophyllous and agree in this respect with most species of Lycopodium. Another feature characteristic of Selaginella, as contrasted with Lycopodium, is the presence of a ligule in both foliage leaves and sporophylls. This is a colourless thin lamina attached by a comparatively stout foot to the base of a pit on the upper surface and close to the lower edge of the leaf ([fig. 130], 4, l; [fig. 131], E, F, l).

Fig. 130. Selaginella grandis. (1–3, nat. size.)

In an erect species, such as S. grandis Moore[114] ([fig. 130] and [fig. 131], G) from Borneo, the main shoots, which may attain a height of 2–3 feet, bear small and inconspicuous leaves of one kind, but the lateral and repeatedly forked shoots are heterophyllous. The passage from the homophyllous to the heterophyllous arrangement is shown in the transition from the erect to the dorsiventral habit of the lateral shoots ([fig. 130], 2). The monopodially or dichotomously branched shoots produce long naked axes at the forks; these grow downwards to the ground where they develop numerous dichotomously forked branches. For certain reasons these naked aerial axes were named rhizophores and have always been styled shoots, the term root being restricted to repeatedly forked branches which the rhizophores produce in the soil. It has, however, been shown by Professor Harvey-Gibson[115] that there is no sufficient reason for drawing any morphological distinction between rhizophores and roots, the term root being applicable to both.

Our knowledge of the anatomy of Selaginella, thanks chiefly to the researches of Harvey-Gibson[116], is much more complete than in the case of Lycopodium. The stems, which may be either trailing or erect, are usually dorsiventral, and it is noteworthy that different shoots of the same plant or even the same axis in different regions may exhibit considerable variation in the structure and arrangement of the vascular tissue. In the well-known species, Selaginella Martensii, the stem, which is partly trailing, partly ascending, possesses a single ribbon-shaped stele composed of scalariform tracheids with two marginal protoxylems formed by the fusion of the leaf-traces of the dorsal and ventral leaves respectively. As in Lycopodium the metaxylem tracheae are as a rule scalariform, but reticulate xylem elements are by no means unknown. The tracheal band, surrounded by parenchymatous elements, is enclosed by phloem with external protophloem elements. The characteristic features of the stele are shown in the diagrammatic drawing of a section of another species—S. Willdenowii—represented in [fig. 131], A.

Fig. 131.

  1. Selaginella Willdenowii. Transverse section of stem: a, outer cortex; p, phloem; t, trabeculae.
  2. S. spinosa, stem: px, protoxylem.
  3. S. laevigata var. Lyallii, section of stele: t, ridge of xylem cylinder; e, endodermis.
  4. S. rupestris, seedlings with cotyledons (c) protruding beyond the sporophylls (b).
  5. Transverse section of Selaginella leaf-base: l, ligule; lt, leaf-trace.
  6. Portion of G. enlarged.
  7. S. grandis. Longitudinal section of strobilus: bb, sporophyll-trace; l, ligule.

A pericycle composed of one or two layers of chlorophyll-containing cells encircles the whole stele which is suspended in a lacuna by trabeculae ([fig. 131], A, B, t) connecting the pericycle with the inner edge of the broad cortex. The trabeculae consist in part of endodermal cells characterised by cuticular bands. The cortex is usually differentiated into three fairly distinct regions. Mechanical tissue of thick-walled fibres constitutes the outer region (a); the middle cortex consists of thinner-walled parenchyma, the elements of which become smaller and rather more compactly arranged in the inner zone. The middle cortex is frequently characterised by the presence of spaces and by the hyphal or trabecular structure of the tissue, a feature which, as Bower[117] pointed out, is common to many recent and fossil members of the Lycopodiales. In some cases, e.g. S. erythropus, from tropical America, the cortex of the creeping stem consists entirely of thick-walled cells. Selaginella grandis ([fig. 130]) has “a short decumbent stem rooted at close intervals[118],” from which thick erect aerial shoots rise to a height of one foot or more. In the apical region these erect axes give off repeatedly forked foliage shoots on which the spiral phyllotaxis of the homophyllous axis is gradually replaced by four rows of two kinds of leaves ([fig. 130], 2). The anatomy of this species agrees with that of S. Martensii. The trailing or semi-erect and homophyllous shoots of Selaginella spinosa[119] present a distinct type of vascular anatomy. The upper part of the ascending stem has an axial strand of xylem with seven peripheral groups of spiral protoxylem tracheae ([fig. 131], B); in the trailing portion of the shoot the protoxylem elements occur as one central group in the solid rod of metaxylem through which the leaf-traces pass on their way to the axial protoxylem. This type is important as affording an exception, in the endarch structure of the xylem, to the usual exarch plan of the stelar tissues. This species is the only one in which any indication of the production of secondary xylem elements has so far been recorded. Bruchmann[120] has shown that, in the small tuberous swelling which occurs at the base of the young shoot (hypocotyl), a meristematic zone is formed round the axial vascular strand and by its activity a few secondary tracheids are added to the primary xylem. With this exception Selaginella appears to have lost the power of secondary thickening, the possession of which constitutes so striking a feature of the Palaeozoic Lycopods. Another type is represented by S. inaequalifolia, an Indian species, the shoots of which may have either a single stele or as many as five, each in its separate lacuna. The homophyllous S. laevigata var. Lyallii Spr., a Madagascan species, affords a further illustration of the variation in plan of the vascular tissues within the genus. There is a considerable difference in structure between the erect and creeping shoots; in the former there may be as many as 12–13 steles, which gradually coalesce before the vertical axis joins the creeping rhizome to form one central and four peripheral steles. In the rhizome there is usually a distinct axial stele without protoxylem, surrounded by an ill-defined lacuna and enclosed by a cylindrical stele (solenostele)[121] usually two tracheae in width with four protoxylem strands on its outer edge. The continuity of the tubular stele is broken and, in transverse section, it assumes the form of a horse-shoe close to the base of an erect shoot to which a crescentic vascular strand is given off. Harvey-Gibson[122] has figured a section of the rhizome of this type in which the axial vascular strand is represented by a slight ridge of tracheae ([fig. 131], C, t) projecting towards the centre of the axis of the tubular stele. The cylindrical stele consists of xylem with external and internal phloem (p): cuticularised endodermal cells occur at e and e.

Reference has already been made to the descending naked branches given off from the points of ramification of the foliage shoots of Selaginella. It has been shown by Harvey-Gibson[123] that these branches, originally designated rhizophores by Nägeli and Leitgeb, as well as the dichotomously branched roots which they produce below the level of the ground, possess a single vascular strand of monarch type. It is interesting to find that in some species the aerial portion of the rhizophore has a xylem strand with a central protoxylem, an instance of endarch structure like that in certain portions of the shoot-system of S. spinosa. The root-anatomy of Selaginella and the dichotomous habit of branching afford points of agreement with the subterranean organs of Lepidodendron and Sigillaria.

Leaves. The leaves of Selaginella[124] usually consist of a reticulum of loosely arranged cells, but in some cases part of the mesophyll assumes the palisade form. The single vascular bundle consists of a few small annular or spiral tracheae and at the apex of the lamina the protoxylem elements are accompanied by several short reticulated pitted elements. Both foliage leaves and sporophylls are characterised by the possession of a ligule, a structure which may present the appearance of a somewhat rectangular plate ([fig. 130], 4, l, and [fig. 131], E–G, l) or assume a fan-shaped form with a lobed or papillate margin. The base, composed of large cells, is sunk in the tissue of the leaf close to its insertion on the stem ([fig. 131], E, l) and enclosed by a well-marked parenchymatous sheath. The sheath is separated from the vascular bundle of the leaf by one or more layers of cells, and in some species these become transformed into short tracheids. The ligule is regarded by Harvey-Gibson[125] as a specialised ramentum which serves the temporary function of keeping moist the growing-point and young leaves.

Cones. The terminal portions of the branches of Selaginella usually bear smaller leaves of uniform size which function as sporophylls, but in this genus the fertile shoots do not generally form such distinct cones as in many species of Lycopodium. In S. grandis (figs. [130], 3; [131], G) the long and narrow strobili consist of a slender axis bearing imbricate sporophylls in four rows: each sporophyll subtends a sporangium situated between the ligule and the axis of the shoot. The sporangium may be developed from the axis of the cone or, as in Lycopodium, from the cells of the sporophyll[126]. In some species the lower sporophylls bear only megasporangia, each normally containing four megaspores, the microsporangia being confined to the upper part of the cone. This distribution of the two kinds of sporangia is, however, by no means constant[127]: in some cases, e.g. S. rupestris, cones may bear megasporangia only, and in the cone of S. grandis, of which a small piece is represented in [fig. 131], G, all the sporangia were found to contain microspores.

The occurrence of two kinds of spores in Selaginella constitutes a feature of special importance from the point of view of the relationship between the Phanerogams, in which heterospory is a constant character, and the heterosporous Pteridophytes. One of the most striking distinctions between the Phanerogams and the rest of the vegetable kingdom lies in the production of seeds. Recent work has, however, shown that seed-production can no longer be regarded as a distinguishing feature of the Gymnosperms and Angiosperms. Palaeozoic plants which combined filicinean and cycadean features resembled the existing Phanerogams in the possession of highly specialised seeds. This discovery adds point to the comparison of the true seed with structures concerned with reproduction in seedless plants, which in the course of evolution gave rise to the more efficient arrangement for the nursing, protection, and ultimate dispersal of the embryo. In the megaspore of Selaginella we have, as Hofmeister was the first to recognise in 1851, a structure homologous with the embryo-sac of the Phanerogam. The embryo-sac consists of a large cell produced in a mass of parenchymatous tissue known as the nucellus which is almost completely enclosed by one or more integuments. Fertilisation of the egg-cell within the embryo-sac takes place as a rule while the female reproductive organ is still attached to the parent-plant and separation does not occur until the ovule has become the seed.

In a few cases, notably in certain plants characteristic of Mangrove swamps, continuity between the seed and its parent is retained until after germination. The megasporangium of Selaginella dehisces[128] along a line marked out by the occurrence of smaller cells over the crest of the wall. It has been customary to describe the megaspores as being fertilised after ejection from the sporangia. This earlier separation from the parent and the absence of any protective covering external to the spore-wall constitute two distinguishing features between seeds and megaspores. In Selaginella apus, a Californian species, Miss Lyon has shown that fertilisation of the egg-cell usually takes place while the megaspore is still in the strobilus. On examining withered decayed strobili of this species which had been partially covered with the soil for some months after fertilisation of the megaspores, several young plants were found with cotyledons and roots projecting through the crevices of the megasporangia[129]. From this, adds Miss Lyon, “it seems safe to assume that an embryo may have two periods of growth separated by one of quiescence quite comparable to those of seed plants with marked xerophilous features.”

In another Western American species S. rupestris described by the same writer the cotyledons of young plants were found protruding from the imbricate sporophylls of a withered cone ([fig. 131], D). This species is interesting also from the occasional occurrence of one instead of four megasporangia in a sporangium; a condition which affords another connecting link between the heterosporous Pteridophytes, on the one hand, and the seed-bearing Phanerogams in which the occurrence of a single embryo-sac (megaspore) in each ovule is the rule. The cones of Selaginella rupestris retain connexion with the plant through the winter and fertilisation occurs in the following spring. After the embryo has been formed the megasporangium “becomes sunken in a shallow pit formed by the cushion-like outgrowth of the sporophyll around the pedicel.” It is suggested that this outgrowth may be comparable with the integument which grows up from the sporophyll in the fossil genus Lepidocarpon[130] and almost completely encloses the sporangium. In the drawings given by Miss Lyon no features are recognisable which afford a parallel to the integument of Lepidocarpon. I have, however, endeavoured to show, by a brief reference to this author’s interesting account of the two Californian species, that the physiological and morphological resemblances between the megasporangia of Selaginella and the integumented ovules of the seed-bearing plants are sufficiently close to enable us to recognise possible lines of advance towards the development of the true seed.

Professor Campbell[131] records an additional example of a Selaginella—probably S. Bigelovii—from the dry region of Southern California in which the spores become completely dried up after the embryo has attained some size, remaining in that state until the more favourable conditions succeeding the dry season induce renewed activity.

Isoetaceae.

The genus Isoetes is peculiar among Pteridophytes both in habit and in anatomical features. In its short and relatively thick tuberous stem, terminating in a crowded rosette of subulate leaves like those of Juncus and bearing numerous adventitious roots, Isoetes presents an appearance similar to that of many monocotyledonous plants. The habit of the genus is well represented by such species as Isoetes lacustris and I. echinospora[132] ([fig. 132]) both of which grow in freshwater lakes in Britain and in other north European countries. The latter species bears leaves reaching a length of 18 cm. The resemblance in habit between this isolated member of the Pteridophytes and certain Flowering plants, although in itself of no morphological significance, is consistent with the view expressed by Campbell that Isoetes may be directly related to the Monocotyledons[133].

Fig. 132. Isoetes echinospora (After Motelay and Vendryès).

  1. Stem of I. lacustris.
  2. Base of sporophyll: l, ligule; spg, sporangium partially covered by velum.

There is as a rule little or no difference between the foliage leaves and sporophylls; in I. lacustris the latter are rather larger and in the terrestrial species I. hystrix[134] the sterile leaves are represented by the expanded basal portions only, which persist like the leaf-bases of Lepidodendron as dark brown scales to form a protective investment to the older part of the stem. The innermost leaves are usually sterile; next to these are sporophylls bearing megasporangia, and on the outside are the older sporophylls with microsporangia. The long and slender portion of the leaf becomes suddenly expanded close to its attachment to the stem into a broad base of crescentic section which bears a fairly conspicuous ligule (figs. [132], B, l, [133], E, l) inserted by a foot or glossopodium in a pit near the upper part of the concave inner face. The ligule is usually larger than that of Selaginella, though of the same type. The free awl-like lamina contains four large canals bridged across at intervals by transverse diaphragms, and in the axial region a single vascular bundle of collateral structure. Other vascular elements, in the form of numerous short tracheids occur below the base of the transversely elongated ligule.

Stomata are found on the leaves of I. hystrix, I. Boryana[135], and in other species which are not permanently submerged. Both microsporangia and megasporangia are characterised by their large size and by the presence of trabeculae or strands of sterile tissue ([fig. 133], E, H, t) completely bridging across the sporangial cavity or extending as irregular ingrowths among the spore-producing tissue. Similar sterile bands, though less abundant and smaller, are occasionally met with in the still larger sporangia of Lepidostrobus; these may be regarded as a further development of the prominent pad of cells which projects into the sporangial cavity in recent species of Lycopodium ([fig. 126], D, p). The sporangia are attached by a very short stalk to the base of a large depression in the leaf-base below the ligule, from the pit of which they are separated by a ridge of tissue known as the saddle, and from this ridge a veil of tissue (the velum) extends as a roof over the sporangial chamber ([fig. 133], E, v). In most species there is a large gap between the lower edge of the velum and that of the sporangial pit, but in I. hystrix this protective membrane is separated from the base of the leaf by a narrow opening, the resemblance of which to the micropyle of an ovule suggested to one of the older botanists the employment of the same term[136]. Mr T. G. Hill[137] has called attention to the presence of mucilage canals in the base of the sporophylls of I. hystrix, which he compares with the strands of tissue known as the parichnos accompanying the leaf-traces of Lepidodendron and Sigillaria in the outer cortex of the stem. The transverse section shown in [fig. 133], H and I, shows two of these mucilage canals in an early stage of development; a strand of parenchymatous elements distinguished by their partially disorganised condition and more deeply stained membranes ([fig. 133], I) runs through the spandrels of the sporophyll tissue close to the upper surface. There is a close resemblance between the structure of these partially formed mucilage-canals and the tissue which has been called the secretory zone in Lepidodendron stems. Fig. 133, H, also shows a large microsporangium with prominent trabeculae (t) lying below the velum. A longitudinal section ([fig. 133], E) through a sporophyll-base presents an appearance comparable with that of an Araucarian cone-scale with its integumented ovule and micropyle. The megaspores are characterised by ridges, spines, and other surface-ornamentation[138]. Though usually unbranched, the perennial stem of Isoetes ([fig. 132]) has in rare cases been found to exhibit dichotomous branching, a feature, as Solms-Laubach[139] points out, consistent with a Lycopodiaceous affinity. The apex is situated at the base of a funnel-shaped depression. The stem is always grooved; in some species two and in others three deep furrows extend from the base up the sides of the short and thick axis towards the leaves: from the sides of these furrows numerous slender roots are given off in acropetal succession. A stele of peculiar structure occupies the centre of the stem; cylindrical in the upper part ([fig. 133], A), it assumes a narrow elliptical or, in species in which there are three furrows, a triangular form in the lower portion of the tuberous stem.

ISOETES

The stem of I. lacustris represented in [fig. 132], A, from which the laminae of the leaves have been removed from the summit affords an example of a species with two furrows. The drawing shows the widely gaping sides of the broad furrow with circular root-scars and a few simple and dichotomously branched roots. A short thick column of parenchymatous tissue projects from a slightly eccentric position on the base of the stem.

Fig. 133. Isoetes lacustris.

The primary vascular cylinder[140] consists of numerous spiral, annular or reticulate tracheids ([fig. 133], A, x) which are either isodiametric or longer in a horizontal than in a vertical direction, associated with parenchyma. Lower in the stem crushed and disorganised xylem elements are scattered through a still living trabecular network of parenchymatous tissue. From the axial cylinder numerous leaf-traces ([fig. 133], A, lt) radiate outwards, at first in a horizontal direction and then gradually ascending towards the leaves. The vascular cylinder is of the type known as cauline; that is, some of the xylem is distinct in origin from that which consists solely of the lower ends of leaf-traces. As in Lycopodium the development of the metaxylem is centripetal.

Von Mohl[141], and a few years later Hofmeister[142], were the first botanists to give a satisfactory account of the anatomy of Isoetes but it is only recently[143] that fresh light has been thrown upon the structural features of the genus the interest of which is enhanced by the many points of resemblance between the recent type and the Palaeozoic Lepidodendreae. A striking anatomical feature is the power of the stem to produce secondary vascular and non-vascular tissue; the genus is also characterised by the early appearance of secondary meristematic activity which renders it practically impossible to draw any distinct line between primary and secondary growth. A cylinder of thin-walled tissue ([fig. 133], A, a) surrounds the primary central cylinder and in this a cambial zone, c, is recognised even close to the stem-apex; this zone of dividing cells is separated from the xylem by a few layers of rectangular cells to which the term prismatic zone has been applied. The early appearance of the cambial activity on the edge of the vascular cylinder is shown in [fig. 133], C, which represents part of a transverse section of a young stem. A leaf-trace, lt, is in connexion with the primary xylem, x′, which consists of short tracheids, often represented only by their spiral or reticulately thickened bands of lignified wall, and scattered parenchyma. Some of the radially elongated cells on the sides of the leaf-trace are seen to be in continuity on the outer edge of the stele, at st, with flattened elements, some of which are sieve-tubes. The position of a second leaf-trace is shown at lt′. External to the sieve-tubes the tissue consists of radially arranged series of rectangular cells, some of which have already assumed the function of a cambium (c). The tissue produced by the cambium on its inner edge consists of a varying amount of secondary xylem composed of very short spiral tracheids; a few of these may be lignified ([fig. 133], A, x2) while others remain thin.

Phloem elements, recognisable by the presence of a thickened reticulum enclosing small sieve-areas (fig 133, B, s) are fairly abundant, and for the rest this intracambial region is composed of thin-walled parenchyma. In longitudinal section these tissues present an appearance almost identical with that observed in a transverse section. Fig. 133, B represents a longitudinal section, through the intracambial zone and the edge of the stele, of a younger stem than that shown in [fig. 133], A. Most of the radially disposed cells internal to the meristematic region are parenchymatous without any distinctive features; a few scattered sieve-tubes (s) are recognised by their elliptical sieve-areas and an occasional tracheid can be detected. The cambium cuts off externally a succession of segments which constitute additional cortical tissue ([fig. 133], A, cr) of homogeneous structure, composed of parenchymatous cells containing starch and rich in intercellular spaces. As the stem grows in thickness the secondary cortex reaches a considerable breadth and the superficial layers are from time to time exfoliated as strips of dead and crushed tissue ([fig. 133], A, b). The diagrammatic sketch reproduced in [fig. 133], A, serves to illustrate the arrangement and relative size of the tissue-regions in an Isoetes stem. In the centre occur numerous spirally or reticulate tracheae scattered in parenchymatous tissue which has been considerably stretched and torn in the peripheral region of the stele; the radiating lines mark the position of the leaf-traces (lt) in the more horizontal part of their course. The zone between the cambium (c) and the edge of the central cylinder consists of radially disposed secondary tissue of short, and for the most part unlignified, elements including sieve-tubes and parenchyma; the secondary xylem elements consist largely of thin-walled rectangular cells with delicate spiral bands, but discontinuous rows of lignified tracheae (x2) occur in certain regions of the intracambial zone. The rest of the stem consists of secondary cortex (cr) with patches of dead tissue (b) still adhering to the irregularly furrowed surface. The structure of the cambium and its products is shown in the detailed drawing reproduced in [fig. 133], D. Many of the elements cut off on the inner side of the cambium exhibit the characters of tracheids: most of these are unlignified, but others have thicker and lignified walls (tr).

I. hystrix appears to be exceptional in retaining its leaf-bases, which form a complete protective investment and prevent the exfoliation of dead cortex. Each leaf-trace consists of a few spiral tracheids accompanied by narrow phloem elements directly continuous with the secondary phloem of the intracambial zone. Dr Scott and Mr Hill have pointed out that a normal cambium is occasionally present in the stem of I. hystrix during the early stages of growth; this gives rise to xylem internally. The few phloem elements observed external to the cambium may be regarded as primary phloem, a tissue not usually represented in an Isoetes stem[144]. The occasional occurrence of this normal cambium, may, as Scott and Hill suggest, be a survival from a former condition in which the secondary thickening followed a less peculiar course. The lower leaf-traces become more or less obliterated as the result of the constant increase in thickness of the broad zone of secondary tissues through which they pass.

The adventitious roots are developed acropetally and arranged in parallel series on each side of the median line of the two or three furrows. The three arms of the triangular stele of I. hystrix and the two narrow ends of the long axis of the stele of I. lacustris, which in transverse section has the form of a flattened ellipse, are built up of successive root-bases. A root of Isoetes ([fig. 133], G) possesses one vascular bundle, x, with a single strand of protoxylem, px, thus agreeing in its monarch structure with the root-bundle in Selaginella and many species of Lycopodium. The cortical region of the root consists of a few layers of outer cortex succeeded by a large space, formed by the breaking down of the inner cortical tissue, into which the vascular bundle projects ([fig. 133], F). The peculiarity of the roots in having a hollow cortex and an eccentric vascular bundle was noticed by Von Mohl[145]. In the monarch bundles, as in the fistular cortex and dichotomous branching, the roots of Isoetes present a striking resemblance to the slender rootlets of the Palaeozoic Stigmaria (see [page 246]). The longitudinal section through the base of a root of Isoetes lacustris shown in [fig. 133], F, affords a further illustration of certain features common to the fossil and recent types.