Fossil plants, Vol. 1: [A text-book] for students of botany and geology

by A. C. Seward

In the example shown in fig. 53 (Equisetum palustre L.[487]) the stout rhizome R gives off from its node, marked by a small and irregular leaf-sheath, two thin roots and a single shoot. The latter has a leaf-sheath at its base, and from the second node, with a larger leaf-sheath, there have been developed branches with tuberous internodes; the constrictions between the tubers and the tips of the terminal tubers bear small leaf-sheaths. Branched roots are also given off from the upper node of the erect shoot.

Near the surface of the ground the buds on the rhizome nodes develope into green erect shoots. The shoot axis is marked out into long internodes separated by nodes bearing the leaf-sheaths. The surface of each internode is traversed by regular and more or less prominent longitudinal ridges and grooves; each ridge marking the position of an internal longitudinal vascular strand. In the axil of each leaf, that is in the axil of each portion of a leaf-sheath corresponding to a marginal uni-nerved tooth, there is produced a lateral bud which may either remain dormant or break through the leaf-sheath and emerge as a lateral branch. At the base of each branch an adventitious root may be formed from a cell immediately below the first leaf-sheath, but in aerial shoots the roots usually remain undeveloped. The lateral branches repeat on a smaller scale the general features of the main axis. In some species, the shoots are unbranched, and in others the slender branches arise in crowded whorls from each node. Leaves, roots and branches are given off in whorls, and the whorls from each node alternate with those from the node next above and next below.

ANATOMY OF EQUISETUM.

In some species of Equisetum the aerial stem terminates in a conical group of sporophylls, while in others the strobilus is formed at the apex of a pale-coloured fertile shoot, which never attains any considerable length and dies down early in the season of growth (fig. 52, A). Below the terminal cone or strobilus there occur one or two modified leaf-sheaths. Such a ring of incompletely developed leaves intervening between the cone of sporangiophores and the normal leaves, is known as the annulus. The annulus is seen in fig. 52, A, immediately below the lowest whorl of sporophylls; it has the form of a low sheath with a ragged margin. In the region of the cone the internodes remain shorter, and the whorls of appendages, known as sporophylls or sporangiophores, have the form of stalked structures terminating distally in a hexagonal peltate disc, which bears on its inner face a ring of five to ten oval sporangia (fig. 52, B). Each sporangium contains numerous spores which eventually escape by the longitudinal dehiscence of the sporangial wall. The opening of the sporangia is probably assisted by the movements of the characteristic elaters formed from the outer wall of each spore.

The spores, which are capable of living only a short time, grow into aerial green prothallia, 1–2 cm. in length; these have the form of irregularly and more or less deeply lobed structures. On the larger and more deeply lobed prothallia the archegonia or female reproductive organs are borne, and the smaller or male prothallia bear the antheridia. On the fertilisation of an egg-cell, the Equisetum plant is gradually developed. For a short time parasitic on the female prothallium or gametophyte, the young plant soon takes root in the ground and becomes completely independent.

As seen in transverse section through a young stem near the apex, the axis consists of a mass of parenchyma, in which may be distinguished a central larger-celled tissue, surrounded by a ring of smaller-celled groups marking the position of a circle of embryonic vascular strands. In each young vascular strand, a few of the cells next the pith may be seen to have thicker walls and to be provided with a ring-like internal thickening; these have passed over into the condition of annular tracheids and represent the protoxylem elements. At a later stage, a transverse section through the stem shows a central hollow pith, formed by the tearing apart and subsequent disappearance of the medullary parenchymatous cells, which were unable to keep pace with the growth in thickness of the stem. The pith cavity is bridged across at each node by a multi-layered plate of parenchyma, which forms the so-called nodal diaphragm. The inner edge of each vascular strand is now found to be occupied by a small irregularly circular canal (fig. 52, C, c, and D, c) in which may be seen some of the rings of protoxylem tracheids (D, a) which have been torn apart and almost completely destroyed. These canals, known as carinal canals, have arisen by the tearing and disruption of the thin-walled cells in the immediate neighbourhood of the protoxylem. Each carinal canal is bounded by a layer of elongated parenchymatous cells which form part of the xylem of the vascular bundle, and is succeeded internally by the general ground-tissue of the stem. The xylem parenchyma next a carinal canal is succeeded externally by phloem tissue, consisting of short protoplasmic cells and longer elements, without nuclei and poor in contents; the latter may be regarded as sieve-tubes. On either side of the phloem, the xylem occurs in two separate bands or groups of annular and reticulately thickened tracheids. In some species, e.g. Equisetum xylochaetum Metten.[488] and E. giganteum[489] L. a native of South America, the xylem has the form of two bands composed of fairly numerous tracheids, but in most species the xylem tracheids occur in small groups, as shown in the figure of E. maximum (fig. 52, D). In the shape of the vascular bundle, and in the formation of the carinal canal, there is a distinct resemblance between the vascular bundles of Equisetum and those of a monocotyledonous stem. These collateral stem-bundles of xylem and phloem traverse each internode as distinct strands, and at the nodes each strand forks into two branches (fig. 54, A), which anastomose with the alternating bundles passing into the stem from the leaf-sheath. Thus the vascular strands of each internode alternate in position with those of the next internode.

Fig. 54. A. Plan of the vascular bundles in the stem of an Equisetum; b, branches passing out to buds (after Strasburger); l, vascular strands passing to the leaf-segments. B. Longitudinal section through a node of E. arvense L. (after Duval-Jouve; × 20). Explanation in the text.

There are certain points connected with the vascular bundles in the nodal region of a shoot, which have an important bearing on the structure of fossil equisetaceous stems. Fig. 54 B represents a diagrammatic longitudinal section through the node of a rhizome of Equisetum arvense from which a root h is passing off in a downward direction, and a branch in an upward direction. The black band c in the parent stem shows the position of the vascular strands; in the region of the node the vascular tissue attains a considerable thickness, as seen at d in the figure. The bands passing out to the left from d go to supply the branch and root respectively. The increased breadth of the xylem strands at the node is due to the intercalation of a number of short tracheids. Fig. 55, 4 shows a transverse section through a mature node of Equisetum maximum; px marks the position of the protoxylem and e that of the endodermis. On comparing this section with that of the internodal vascular bundle in fig. 52, D, the much greater development of wood in the former is obvious; the carinal canal of the internodal bundle is absent in the section through a node. The disposition of the xylem tracheids in fig. 55, 4 shows a certain regularity which, though not very well marked, suggests the development of wood elements as the result of cambial activity. Longitudinal sections through the nodal region demonstrate the existence of “cells similar to those of an ordinary cambium, and a cell-formation resulting from their division which is similar to that in an ordinary secondary thickening.”[490] The short tracheids which make up this nodal mass of xylem differ from those in the internodal bundle in their smaller size, and in being reticulately thickened. There is, therefore, evidence that in the nodes of some Equisetum stems additional xylem elements are produced by a method of growth comparable with the cambial activity which brings about the growth in thickness of a forest-tree[491]. The significance of these statements will be realised when the structure of the extinct genus Calamites is described and compared with that of Equisetum.

The small drawing in fig. 55, 3 shows part of the ring of thick nodal wood; the section cuts through two bundles about their point of bifurcation, the strand x is passing out in a radial direction to a lateral branch, the strand to the right of x and the separate fragment of a strand to the left of x are portions of leaf-trace bundles on their way to the leaf-sheath. Reverting to fig. 54, B, the other structures seen in the section are the leaf-sheaths (l and m), the vallecular canal (f), the epidermis, cortex and pith (k, e and a) of the stem. The epidermis which has been ruptured by the root and branch is indicated at i, i; the dotted lines traversing the upper part of the pith of the lateral branch mark the position of a nodal diaphragm.