III. RADIATE THALLUS
1. CHARACTERS OF RADIATE THALLUS
In the stratose dorsiventral thallus, there is a widely extended growing area situated round the free margins of the thallus. In the radiate thallus of the fruticose or filamentous lichens, growth is confined to an apical region. Attachment to the substratum is at one point only—the base of the plant—thus securing the exposure of all sides equally to light. The cortex surrounds the fronds, and the gonidia (mostly Protococcaceae) lie in a zone or in groups between the cortex and the medulla. It is the highest type of vegetative development in the lichen kingdom, since it secures the widest room for the gonidial layer, and the largest opportunity for photosynthesis.
Fig. 57. Roccella fuciformis DC.
Shrubby upright lichens consist mostly of strap-shaped fronds, either simple or branched, which may be broadened to thin bands ([Fig. 57]) or may be narrowed and thickened till they are almost cylindrical. The fronds vary in length according to the species from a few millimetres upwards: those of Roccella have been found measuring 30 cm. in length; those of Ramalina reticulata, the largest of all the American lichens, extend to considerably more.
Lichens of filamentous growth are more or less cylindrical ([Fig. 58]). They are in some species upright and of moderate length, but in a few pendulous forms they grow to a great length: specimens of Usnea longissima have been recorded that measured 6 to 8 metres from base to tip.
Fig. 58. Usnea barbata Web. (S. H., Photo.).
The radiate type of thallus occurs in most of the lichen groups but most frequently in the Gymnocarpeae. In gelatinous Discolichens it is represented in the Lichinaceae. It is rare among Pyrenocarpeae: there is one very minute British lichen in that series, Pyrenidium actinellum, and one from N. America, Pyrenothamnia, that are of fruticose habit.
2. INTERMEDIATE TYPES OF THALLUS
Between the foliose and the fruticose types, there are intermediate forms that might be, and often are, classified now in one group and now in the other. These are chiefly: Physcia (Anaptychia) ciliaris, Ph. leucomelas and the species of Evernia.
In the two former the habit is more or less fruticose as the plants are affixed to the substratum at a basal point, but the fronds are decumbent and the internal structure is of the dorsiventral type: there is an upper “fibrous” cortex of closely compacted parallel hyphae, a gonidial zone—the gonidia lying partly in the cortex and partly among the loose hyphae of the medulla—and a lower cortex formed of a weft of hyphae which also run somewhat parallel to the surface. Both species are distinguished by the numerous marginal cilia, either pale or dark in colour. These two lichens are greyish-coloured on the upper surface and greyish or whitish below.
Evernia furfuracea with a basal attachment[374], and with a partly horizontal and partly upright growth, has a dorsiventral thallus, dark greyish-green above and black beneath, with occasional rhizinae towards the base. The cortex of both surfaces belongs to the “decomposed” type; the gonidial zone lies below the upper surface, and the medullary tissue is of loose hyphae. In certain forms of the species isidia are abundant on the upper surface, a character of foliose rather than of fruticose lichens. E. furfuracea grows on trees and very frequently on palings.
Fig. 59. Evernia prunastri Ach. (M. P., Photo.).
E. prunastri, the second species of the genus, is more distinctly upright in habit, with a penetrating basal hold-fast and upright strap-shaped branching fronds, light-greyish green on the “upper” surface and white on the other ([Fig. 59]). The internal structure is sub-radiate; both cortices are “decomposed”; the gonidial zone consists of somewhat loose groups of algae, very constant below the “upper” surface, with an occasional group in the pith near to the lower cortex in positions that are more exposed to light. There is also a tendency for the gonidial zone to pass round the margin and spread some way along the under side. The medulla is of loose arachnoid texture and the whole plant is very limp when moist. It grows on trees, often in dense clusters.
3. FRUTICOSE AND FILAMENTOUS
A. General Structure of Thallus
The conditions of strain and tension in the upright plant are entirely different from those in the decumbent thallus, and to meet the new requirements, new adaptations of structure are provided either in the cortex or in the medulla.
Cortical Structures. With the exception of the distinctly plectenchymatous cortex, all the other types already described recur in fruticose lichens; in various ways they have been modified to provide not only covering but support to the fronds.
a. The fastigiate cortex. This reaches its highest development in Roccella in which the branched hyphal tips, slightly clavate and thick-walled, lie closely packed in palisade formation at right angles to the main axis ([Fig. 45]). They afford not only bending power, but give great consistency to the fronds. The cortex is further strengthened in R. fuciformis[375] by the compact arrangement of the medullary hyphae that run parallel with the surface, and among which occur single thick-walled filaments. The plant grows on maritime rocks in very exposed situations; and the narrow strap-shaped fronds, as stated above, may attain a length of 30 cm., though usually they are from 10 to 18 cm. in height. The same type of cortex, but less highly differentiated, affords a certain amount of stiffness to the cylindrical much weaker fronds of Thamnolia.
b. The fibrous cortex. This type is found in a number of lichens with long filamentous hanging fronds. It consists of parallel hyphae, rarely septate and rarely branched, but frequently anastomosing and with strongly thickened “sclerotic” walls. Such a cortex is the only strengthening element in Alectoria, and it affords great toughness and flexibility to the thong-like thallus. It is also present in Ramalina (Alectoria) thrausta, a species with slender fronds ([Fig. 60]).
Fig. 60. Alectoria thrausta Ach. A, transverse section of frond; a, cortex; b, gonidia; c, arachnoid medulla × 37. B, fibrous hyphae from longitudinal section of cortex. × 430 (after Brandt).
In Usnea longissima the cortex both of the fibrillose branchlets and of the main axis is fibrous, and is composed of narrow thick-walled hyphae which grow in a long spiral round the central strand. The hyphae become more frequently septate further back from the apex ([Fig. 61]). Such a type of cortex provides an exceedingly elastic and efficient protection for the long slender thallus.
Fig. 61. Usnea longissima Ach. Longitudinal sections of outer cortex. A, near the apex; B, the middle portion of a fibril. × 525 (after Schulte).
The same type of cortex forms the strengthening element in the fruticose or partly fruticose members of the family Physciaceae. One of these, Teloschistes flavicans, is a bright yellow filamentous lichen with a somewhat straggling habit. The fronds are very slender and are either cylindrical or slightly flattened. The hyphae of the outer cortex are compactly fibrous; added toughness is given by the presence of some longitudinal strands of hyphae in the central pith.
Another still more familiar grey lichen, Physcia ciliaris, has long flat branching fronds which, though dorsiventral in structure, are partly upright in habit. Strength is secured as in Teloschistes by the fibrous upper cortex. Other species of Physciae are somewhat similar in habit and in structure.
In Dendrographa leucophaea, a slender strap-shaped rock lichen, Darbishire[376] has described the outer cortex as composed of closely compacted parallel hyphae resembling the strengthening cortex of Alectoria and very different from the fastigiate cortex of the Roccellae with which it is usually classified.
B. Special strengthening Structures
a. Sclerotic strands. This form of strengthening tissue is characteristic of Ramalina. With the exception of R. thrausta (more truly an Alectoria) all the species have a rather weak cortical layer of branching intricate thick-walled hyphae, regarded by Brandt[377] as plectenchymatous, but more correctly by Hue[378] as “decomposed” on account of the gelatinous walls and diminishing lumen of the irregularly arranged cells.
Fig. 62. Ramalina minuscula Nyl. A, transverse section of frond × 37; B, longitudinal strengthening hyphae of inner cortex × 430 (after Brandt).
In R. evernioides, a plant with very wide flat almost decumbent fronds of soft texture, in R. ceruchis and in R. homalea there is a somewhat compact medulla which gives a slight stiffness to the thallus. The other species of the genus are provided with strengthening mechanical tissue within the cortex formed of closely united sclerotic hyphae that run parallel to the surface ([Fig. 62]). In a transverse section of the thallus, this tissue appears sometimes as a continuous ring which may project irregularly into the pith (R. calicaris); more frequently it is in the form of strands or bundles which alternate with the groups of gonidia (R. siliquosa, R. Curnowii, etc.). In R. fraxinea these strands may be scarcely discernible in young fronds, though sometimes already well developed near the tips. Occasionally isolated strands of fibres appear in the pith (R. Curnowii), or the sclerotic projections may even stretch across the pith to the other side (R. strepsilis) ([Fig. 75 B]).
In the Cladoniae support along with flexibility is secured to the upright podetium by the parallel closely packed hyphae that form round the hollow cylinder a band called the “chondroid” layer from its cartilage-like consistency.
b. Chondroid axis. The central medullary tissue in Ramalina is, with few exceptions, a loose arachnoid structure; often the fronds are almost hollow. In one species of Usnea, U. Taylori, found in polar regions, there is a similar loose though very circumscribed medullary and gonidial tissue in the centre of the somewhat cylindrical thallus, and a wide band of sclerotic fibres towards the cortex.
Fig. 63 A. A, Usnea barbata Web. Longitudinal section of filament with young adventitious branch. a, chondroid axis; b, gonidial tissue; c, cortex. × 100 (after Schwendener). B, U. longissima Ach. Hyphae from central axis × 525 (after Schulte).
In all other species of Usnea the medulla itself is transformed into a strong central strand of long-celled thick-walled hyphae closely knit together by frequent anastomoses ([Fig. 63 A]). This central strand of the Usneas is known as the “chondroid axis.” A narrow band of loose air-containing hyphae and a gonidial zone lie round the central axis between it and the outer cortex ([Fig. 63 A], b). At the extreme apex, the external cortical hyphae grow in a direction parallel with the long axis of the plant, but further back, they branch out at right angles and become swollen and mostly “decomposed” as in the cortex of Ramalina.
In Letharia (L. vulpina, etc.) the structure is midway between Ramalina and Usnea: the central axis is either a solid strand of chondroid hyphae or several separate strands.
Fig. 63 B. Usnea longissima Ach. A, transverse section of fibril × 85. B, a, chondroid axis; b, gonidial tissue; c, cortex × 525 (after Schulte).
In three other genera with upright fruticose thalli, Sphaerophorus, Argopsis and Stereocaulon, rigidity is maintained by a medulla approaching the chondroid type. In Sphaerophorus the species may have either flattened or cylindrical branching stalks, but in all of them, the centre is occupied by longitudinal strands of hyphae. Argopsis, a monotypic genus from Kerguelen, has a cylindrical branching thallus with a strong solid axis; it is closely allied to Stereocaulon, a genus of familiar moorland lichens. The central tissue of the stalks in Stereocaulon is also composed of elongate, thick-walled conglutinate hyphae, formed into a strand which is, however, not entirely solid.
C. Survey of Mechanical Tissues
Mechanical tissues scarcely appear among fungi, except perhaps as stoutish cartilaginous hyphae in the stalks of some Agarics (Collybiae, etc.), or as a ring of more compact consistency round the central hyphae of rhizomorphic strands. It is practically a new adaptation of hyphal structure confined to lichens of the fruticose group, where there is the same requirement as in the higher plants for rigidity, flexure and tenacity.
Rigidity is attained as in other plants by groups or strands of mechanical tissue situated close to the periphery, as they are so arranged in Ramalina and Cladonia; or the same end is achieved by a strongly developed fastigiate cortex as in Roccella. Bending strains to which the same lichens are subjected, are equally well met by the peripheral disposition of the mechanical elements.
Tenacity and elasticity are provided for in the pendulous forms either by a fibrous cortex as in Alectoria, or by the chondroid axis in Usnea. Haberlandt[379] has recorded some interesting results of tests made by him as to the stretching capacity of a freshly gathered pendulous species in which the central strand was from ·5 to 1 mm. thick. He found he could draw it out 100 to 110 per cent. of its normal length before it gave way. In an upright species the frond broke when stretched 60 to 70 per cent. In both of the plants tested, the central strand retained its elasticity up to 20 per cent. of stretching. The outer cortical tissue was cracked and broken in the experiments. Schulte[380] calculated somewhat roughly the tenacity of Usnea longissima and found that a piece of the main axis 8 cm. long carried up to 300 grms. without breaking.
D. Reticulate Fronds
In the upright radiate thallus, more especially among the Ramalinae, though also among Cladoniae[381], there has appeared a reticulate thallus resulting from the elongate splitting of the tissues, and due to unequal growth tension and straining of the gelatinous cortex when swollen with moisture. In several species of Ramalina, the strap-shaped frond is hollow in the centre; and strands of strengthening fibres give rise to a series of cortical ridges. The thinner tissue between is frequently torn apart and ellipsoid openings appear which do not however pierce beyond the central hollow. Such breaks are irregular and accidental though occurring constantly in Ramalina fraxinea, R. dilacerata, etc.
A more complete type of reticulation is always present in a Californian lichen, Ramalina reticulata, in which the large flat frond is a delicate open network from tip to base ([Fig. 64]). It grows on the branches of deciduous trees and hangs in crowded tufts up to 30 cm. or more in length. Usually it is so torn, that the real size attainable can only be guessed at. It is attached at the base by a spreading discoid hold-fast, and, in mature plants, consists of a stoutish main axis from which side branches are irregularly given off. These latter are firm at the base like the parent stalk, but soon they broaden out into very wide fronds. Splitting begins at the tips of the branches while still young; they are then spathulate in form with a slightly narrower recurved tip, below which the first perforations are visible, small at first, but gradually enlarging with the growth of the frond.
Fig. 64. Ramalina reticulata Krempelh. Portion of frond (after Cramer).
Ramalina reticulata is an extremely gelatinous lichen and the formation of the network was supposed by Lutz[382] to be entirely due to the swelling of the tissues, or the imbibition of water, causing tension and splitting. A more exact explanation of the phenomenon is given by Peirce[383]: he found that it was due to the thickened incurved tip, which, on the addition of moisture, swells in length, breadth and thickness, causing it to bend slightly upwards and then curve backwards over the thallus, thus straining the part immediately behind. These various movements result in the splitting of the frond while it is young and the cortices are thin and weak.
Peirce made a series of experiments to test the capacity of the tissues to support tensile strains. In a dry state, a piece of the lichen held a weight up to 150 grms.; when wet it broke with a weight of 30 grms. It was also observed that the thickness of the frond doubled on wetting.
E. Rooting Base in Fruticose Lichens
Fruticose and filamentous lichens are distinguished by their mode of attachment to the substratum: instead of a system of rhizinae or of hairs spread over a large area, there is usually one definite rooting base by which the plant maintains its hold on the support.
Intermediate between the foliose and fruticose types of thallus are several species which are decumbent in habit, but which are attached at one (or sometimes more) definite points, with but little penetration of the underlying substance. One such lichen, Evernia furfuracea, has been classified now as foliose, and again as fruticose. The earliest stage of the thallus is in the form of a rosette-like sheath which bears rhizinae on the under surface, very numerous at the centre of the sheath, but entirely wanting towards the periphery. A secondary thallus of strap-shaped rather narrow fronds rises from the sheath and increases by irregular dichotomous branching. These branches, which are considered by Zopf[384] as adventitious, may also come into contact with the substratum and produce a few rhizinae at that point; or if the frond is more closely applied, the irritation thus produced causes a still greater outgrowth of rhizinae and the formation of a new base from which other fronds originate. These renewed centres of growth are not of very frequent occurrence; they were first observed and described by Lindau[385] in another species, Evernia prunastri, and were aptly compared by him to the creeping stolons of flowering plants.
Evernia furfuracea grows frequently on dead wood, palings, etc., as well as on trees. E. prunastri grows invariably on trees, and has a more constantly upright fruticose habit; in this species also, a basal sheath is present, and the attachment is secured by means of rhizoidal hyphae which penetrate deeply into the periderm of the tree, taking advantage of the openings afforded by the lenticels. The sheath hyphae are continuous with the medullary hyphae of the frond, and gonidia are frequently enclosed in the tissues; the sheath spreads to some extent over the surface of the bark, and round the base of the fronds, thus rendering the attachment of the lichen to the tree doubly secure.
Among Ramalinae, the development of the base was followed by Brandt[386] in one species, R. Landroensis, an arboreal lichen from S. Tyrol. A rosette-like sheath was formed consisting solely of strands of thick-walled hyphae which spread over the bark. There were no gonidia included in the tissue.
A different type of attachment was found by Lilian Porter[387] in corticolous Ramalinae—R. fraxinea, R. fastigiata, and R. pollinaria. The lichens were anchored to the tree by strands of closely compacted hyphae longitudinally arranged and continuous with the cortical hyphae. These enter the periderm of the tree by cracks or lenticels, and by wedge action cause extensive splitting. The strands may also spread horizontally and give rise to new plants. The living tissues of the tree were thus penetrated and injured, and there was evidence that hypertrophied tissue was formed and caused erosion of the wood.
Fig. 65. Ramalina siliquosa A. L. Sm., on rocks, reduced (M. P., Photo.).
Several Ramalinae—R. siliquosa, R. Curnowii, etc.—grow on rocks, often in extremely exposed situations, in isolated tufts or in crowded swards ([Fig. 65]). The separate tufts are not unfrequently connected at the base by a crustaceous thallus. It is possible also to see on the rock, here and there, small areas of compact thalline granules that have scarcely begun to put out the upright fronds. These granules are corticate on the upper surface and contain gonidia; from the lower surface, slender branching hyphae in rhizoid-like strands penetrate down between the inequalities and separable particles of the rock, if the formation is granitic. They frequently have groups of gonidia associated with them, and they continue to ramify and spread, the pure white filaments often enough enclosing morsels of the rock. The upright fronds are continuous with the base and are thus securely anchored to the substratum.
On a smooth rock surface such as quartzite a continuous sward of Ramalina growth is impossible. The basal hyphae being unable to penetrate the even surface of the rock, the attachment is slight and the plants are easily dislodged. They do however succeed, sometimes, in taking hold, and small groups of fronds arise from a crustaceous base which varies in depth from ·5 to 1 mm. The tissues of this base are very irregularly arranged: towards the upper surface loose hyphae with scattered groups of algae are traversed by strands of gelatinized sclerotic hyphae similar to the strengthening tissues of the upright fronds, while down below there are to be found not only slender hyphae, but a layer of gonidia visible as a white and green film on the rock when the overlying particles are scaled off.
Darbishire[388] found that attachment to the substratum by means of a basal sheath was characteristic of all the genera of Roccellaceae. He looks on this sheath, which is the first stage in the development of the plant, as a primary or proto-thallus, analogous to the primary squamules of the Cladoniae, and he carries the analogy still further by treating the upright fronds as podetia. The sheath of the Roccellaceae varies in size but it is always of very limited extent; it is mainly composed of medullary hyphae, and gonidia may or may not be present. The whole structure is permanent and important, and is generally protected by a well-developed upper cortex similar in structure to that of the upright thallus, i.e. of a fastigiate type. There is no lower cortex.
The two British species of Roccella—R. fuciformis and R. phycopsis—grow on maritime rocks, the latter also occasionally on trees. In R. fuciformis, the attaching sheath is a flat structure which slopes up a little round the base of the upright frond. It is about 2 mm. thick, the cortex occupying about 40 µ of that space; a few scattered gonidia are present immediately below. The remaining tissue of the sheath is composed of firmly wefted slender filaments. Towards the lower surface, there is a more closely compacted dark brown layer from which pass out the hyphae that penetrate the rock.
The sheath of R. phycopsis is a small structure about 3 to 4 mm. in width and 1·5 mm. thick. A few gonidia may be found below the dense cortical layer, but they tend to disappear as the upright fronds become larger and the shade, in consequence, more dense. Lower down the hyphae take an intensely yellow hue; mixed with them are also some brown filaments. A somewhat larger sheath 7 to 8 mm. wide forms the base of R. tinctoria. In structure it corresponds—as do those of the other species—with the ones already described.
In purely filamentous species such as Usnea there is also primary sheath formation: the medullary hyphae spread out in radiating strands which force their way wherever possible into the underlying substance; on trees they enter into any chink or crevice of the outer bark like wedges; or they ramify between the cork cells which are split up by the mere growth pressure. By the vertical increase of the base, the fronds may be hoisted up and an intercalary basal portion may arise lacking both gonidia and cortical layer. Very frequently several bases are united and the lichen appears to be of tufted habit.
A basal sheath provides a similar firm attachment for Alectoria jubata and allied species: these are slender mostly dark brown lichens which hang in tangled filaments from the branches of trees, rocks, etc.
These attaching sheaths differ in function as well as in structure from the horizontal thallus of the Cladoniaceae. They may be more truly compared with the primary thallus of the red algae Dumontia and Phyllophora which are similarly affixed to the substratum, while upright fronds of subsequent formation bear the fructifications.