A handbook of systematic botany

by Eugenius Warming

Selaginella (300–400 species), is essentially tropical, only one species living in the North (S. spinulosa), but others grow in Central and South Europe.

Order 3. Lepidodendraceæ are extinct, tree-like Lycopods, which are found especially in the Lower and Middle Carboniferous. Vegetatively they are most nearly related to Lycopodium, but the stem attained much larger proportions (about eleven metres in height and one metre in thickness), and had a cambium by which it increased in thickness. It was regularly dichotomous, and closely studded with spirally-placed leaves, which left behind them peculiar rhombic scars. The large cones resemble Pine-cones, and bore sporangia much larger than any which are now produced (the male ones as much as 2 cm.’s in length). The macrosporangia were situated at the base, and the microsporangia at the apex.

Order 4. Sigillariaceæ. These are, presumably, another group of extinct tree-like Lycopods (especially in the Middle Carboniferous). The name has been derived from the seal-like scars, which the fallen leaves have left behind in longitudinal rows on the grooved stem. The rhizomes of these plants were formerly termed Stigmaria, and placed in a separate genus.

Order 5. Sphenophyllaceæ form an entirely extinct group. They do not definitely belong to any of the three large classes of Vascular Cryptogams, but it is perhaps best to place them in juxtaposition to these. They were herbaceous plants with verticillate, wedge-shaped leaves, with nerves branching dichotomously into equally strong branches. Micro-and macrosporangia were formed in the same cone; and were situated in the axils of the leaves, as in the Lycopods.

The Transition from the Cryptogams to the Phanerogams.

All the plants considered in the preceding chapters are included in the term Cryptogams; all in the following chapters under the head of Phanerogams (see page [3]). Hofmeister’s pioneer works (1851, Vergleichende Untersuchungen der höheren Kryptogamen, etc.) and the numerous researches published later by other investigators, have closed the gap which was formerly thought to exist between these plants; so that we now, in the series: Bryophyta—Pteridophyta—Gymnospermæ—Angiospermæ see the expression of a single line of development in accordance with a definite plan. The forms through which this gradual development has taken place have in course of time, however, to a great extent died out, and only single links of the chain connecting the lowest to the highest still remain.

The alternation of generations, which we found indicated in certain Thallophytes, can be proved with the greatest clearness in all the higher Cryptogams, from the Mosses upwards; it is also found in the Phanerogams, but not in such a pronounced degree, because one of the generations is so far reduced that it has almost given up its independence. For the sake of greater clearness, we will begin with the comparison of the sporophyte, asexual (second) generation.

The asexual (2nd) generation of the Cormophytes.

The asexual generation which follows from the further development of the fertilised oosphere, is, in the Mosses, only the sporogonium (according to one theory it is perhaps homologous with a spore-bearing leaf, situated upon a short stem, see p. [187]); in Filicinæ, Equisetinæ, and Lycopodinæ, on the other hand, it is a highly developed plant differentiated into stem, leaf, and true root, and bearing the sporangia on its leaves. The MODIFICATION OF THE SHOOT is very slight in Filicinæ. The first leaves of the embryo are very simple in form (Fig. [205]), but after a certain age all the leaves which arise are essentially alike. The fertile leaves do not differ from the barren ones, and are found associated with them, and their formation does not limit the growth in length of the stem. It is only in a few of the true Ferns, and in the Hydropterideæ, that the fertile leaves differ considerably from the barren ones. A division of labour in which certain leaves are set apart for nutrition, and others for reproduction, is found more pronouncedly in the Equisetinæ and Lycopodinæ, for in these groups, with a few exceptions, the fertile and barren leaves are very dissimilar; the former are collected in special ear-like cones, which terminate the further growth of the short stems on which they are borne. In connection with the cone, leaves are sometimes developed which form a transition from the barren to the fertile ones (the “annulus” in Equisetaceæ), and in these cases the first indication of a flower with perianth or floral-leaves is to be traced. Among the Cryptogams the highest division of labour is found in Selaginella and Isoëtes, which have the two kinds of sporangia borne on different leaves. The division of labour (modification) is, however, still more pronounced in the Phanerogams: the leaves which bear the microsporangia (“pollen-sacs”) have quite different forms from those which bear macrosporangia (the “nucellus” in the ovule), the former are termed stamens, the latter carpels; in certain instances, too, there is even a contrast between the “male plants” and the “female plants.” Moreover, a division of labour, in a much greater degree, takes place in the leaves which do not directly take part in reproduction, and it is thus possible in many plants to draw a sharp line not only between stamens and carpels, but also between four or five distinct kinds of leaves, which differ in form, structure, and corresponding functions, and which appear in regular sequence on the shoot: namely, between “scale-leaves” and “foliage-leaves,”[21] both of which occur in the Cryptogams, and the “floral-leaves,” including the bracts and leaves of the “perianth,” which latter often differ from each other in form and colour, and are then separated into sepals and petals. The leaves—stamens and carpels—which bear the sporangia are termed sporophylls, and the shoot, or extremity of a shoot, whose leaves are modified into sporophylls, is terminated in its further growth by their production, and is known as a flower. The flower which is most completely furnished has calyx, corolla, stamens, and carpels arranged in this order. If the only sporophylls present are stamens, then it is said to be a male (staminate) flower, and if only carpels, then a female (pistillate) flower, and in both these cases the flowers are unisexual, or diclinous. If stamens and carpels are both present in the same flower, it is termed hermaphrodite. Diclinous plants in which the female flowers are situated on one plant, and the male flowers on another, are termed diœcious; and those in which the same plant bears the two kinds of flowers are termed monœcious. When the male, female, and hermaphrodite flowers are found in the same species, the plant is said to be polygamous.

The sporangia-bearing leaves—Sporophylls. In the Mosses the asexual generation is only represented by the sporogonium, and if the theory is correct which considers the sporogonium to be an embryo consisting of a rudimentary stem and terminal leaf, then the spores are produced on the leaves in these plants. The sporangia in the Filicinæ are situated in groups (sori) on the back or on the edge of the leaves. The number of sporangia in the sorus diminishes very greatly in the Marattiaceæ and Gleicheniaceæ (three to four in the latter, Fig. [213]). In the Equisetinæ the sporangia are situated in a small number on the underside of shield-like leaves, and in Lycopodinæ, singly, in the axils of the fertile leaves, which are alike and bear either micro- or macrosporangia. In the Phanerogams there is a great difference between the stamens and carpels.