A handbook of systematic botany

by Eugenius Warming

Fig. 224.—Equisetum arvense: a fertile branch with cone; b vegetative shoot; c cone; d sporophylls.

The vegetative aerial STEMS are divided into a number of internodes by the whorls of leaves (Fig. [224]). The internodes are hollow, the cavities being separated from each other by the transverse partitions of the solid nodes. The lower portion of the internode, which is encased by the leaves, has much thinner and softer cell-walls, so that the stem is easily separated into segments just above the nodes. Each internode has a large number of ridges and furrows, and bears at its apex a whorl of leaves whose number and position correspond to the ridges of the internode. As in the case of other verticillate plants, the whorls are placed alternately, one above the other; the same arrangement is also found in the ridges on two successive internodes. In addition to the large air-cavity in the centre of each internode (the central cavity), a whorl of tubular air-passages is found in the cortex of the stems, opposite the furrows (vallecular canals). There is also a similar air-passage (carinal canals) in each of the vascular bundles, which are placed in a ring, one opposite each ridge, and therefore alternating with the vallecular canals. The vascular bundles are collateral as in the majority of Flowering-plants, but poorly developed. The xylem of each bundle consists of two groups of annular or spiral vessels, close to the outer border of the carinal canal, and two groups of scalariform tracheides, each placed on a radius passing through a group of spiral vessels. The phloëm is placed between these four groups, each of which has only a few vessels. The stiffness of the stems is mainly due to the large amount of silica in the cell-walls of the epidermis, and to the sclerenchymatous cells of the ridges.

All LEAVES are situated in whorls. The VEGETATIVE are simple, undivided, 1-nerved, and are united into toothed sheaths (Fig. [224] a, b). The branching of the stems in some species (E. arvense) is very abundant. The branches break through the base of the leaf-sheaths (Fig. [224] b), and generally alternate with the teeth (leaves).

The FERTILE LEAVES (sporophylls) are different from the barren ones. They are free, shield-like, each one having a short stalk bearing usually an hexagonal plate (Fig. [224] d), and closely compressed into an ear or cone (Fig. [224] a, c). The Equisetums thus present an advance in development distinctly beyond that of the Ferns, which is further emphasized by the circumstance that a transition from the sheath-leaves to the fertile-leaves is found in the involucre or annulus, a “collar” of specially modified leaves situated at the base of the cone (Fig. [224] a and c). The cone may be considered as a very rudimentary flower, and the annulus may be regarded as a very early stage in the formation of a flower (perianth). See page [235].

The SPORANGIA are situated on the underside of the sporophylls, one at each angle; they are sac-like, and open inwardly by a longitudinal cleft (Fig. [224] d). An annulus is wanting; but in the wall of the sporangium, as in the pollen-sacs of the Flowering-plants, a layer of cells, with annular or spiral thickenings, is developed, which assists in the dehiscence of the sporangium.

The SPORES are green; the walls composed of four distinct layers, of which the outer is gradually separated, except at one point, and becomes split into four long bands (elaters) (Fig. [225]). The elaters are extremely hygroscopic, coiling round the spore when moistened, and expanding as soon as dry, presenting a most lively object under the microscope when breathed upon and allowed to dry. The second layer, when germination commences, becomes detached from the inner wall, which is formed of the exospore and endospore.

The order has become much reduced, and at the present time includes only one genus, Equisetum, with about twenty-five species, which are distributed over the entire globe, particularly in damp situations. In SOME SPECIES the barren shoots are green and very much branched, but the fertile ones are unbranched, pale brown, and possess no chlorophyll (E. arvense, Field-Horsetail, Fig. [224], and E. maximum). In others the fertile and barren shoots are alike green, and either both unbranched (E. hiemale), or branched (E. palustre, E. limosum, etc). The fertile shoots of E. silvaticum, up to maturity, resemble those without chylorophyll of E. arvense, but after that period they produce green branches, and thus resemble the barren ones.

Fig. 225.—Spores of Equisetum: A damp, with elaters (e) coiled round the spore; B dry, with elaters expanded.

Extinct isosporous Equisetinæ. In addition to several true species of fossilized Equisetums, the order of the Calamites, which no doubt is closely allied to the Equisetinæ, is also found in the fossil state. These were gigantic forms, attaining about twenty times the size of those of the present day, and stems of nearly 10–12 metres in height are known. They reached the culminating point of their development in the Carboniferous period, and died out towards the close of the Palæozoic. The stems had hollow internodes and alternating grooves, similar to their relatives of the present day. The leaves must either have been absent or very perishable, since they have not been identified with certainty. If the determinations of certain remains of cones which of late have been discovered are correct, they were heterosporous and had two kinds of sporangia as in the following sub-class. A cambium formation and an increase in thickness has been found in the stems.