Draw a well labelled diagram of internal structure of Equisetum stem and mention its salient features.

Ans. The stem is covered over by a single layered epidermis interrupted by stomata situated in the grooves of furrows. The epidermis is always impregnated with a thick layer of silica. The deposition of the silica on the epidermal layer gives the rough appearance to the stem, and therefore, the Equisetum plants are also known as scouring rushes’. The stomata are found in grooves of the aerial shoots. The development of the stomata is somewhat peculiar in the sense that the initial divides twice by successive longitudinal divisions and this way the two innermost cells develop into the guard cells, whereas, the two outermost cells develop into accessory cells on being mature of the stomata, the two accessory cells completely overarch the guard cells and the stoma. In majority of the species e.g. E. hymeale, E. ramosissimum etc., the stomata are sunken, whereas in some other species, e. g. E. palustre, E. pratense etc., they are situated on the surface of the epidermis. The silica is deposited in the wall of guard cells in transverse radial bands.

Just underneath the epidermis, there is broad cortex. The cortex consists of mechanical and assimilatory tissue. In the outer cortex, just beneath each of the ridges, there is a strand of sclerenchyma. Usually the sclerenchyma is restricted to the periphery but in E. giganteum it extends inward to the endodermis. These columns of sclerenchyma are chief mechanical elements of the shoot. In addition to the above mentioned sclerenchymatous columns, the sclerenchyma strands are also found in the furrows. Here, each strand is situated in between the curved strands of chlorenchyma. The chlorenchyma possess well developed intercellular spaces. These spaces are very much distinct below the stomata. These chlorenchymatous bands are found beneath the sclerenchymatous strands situated underneath the ridges. The ends of the chlorenchymatous bands touch the epidermis in the grooves. Since the leaves are small, scaly and have less number of chloroplasts the chlorenchyma of the cortex of the shoot is the major photosynthetic tissue. The inner cortex is composed of thin walled parenchyma with well developed intercellular spaces. In this region of the cortex the air spaces known as vallecular canals are also present. Usually vallecular canals are found opposite the furrows in the deeper tissue of the cortex. They are alternate to the vascular bundles. These canals are filled up with air.

The endodermis is the last layer of the cortex. In E. arvense, E. telmateia, E. palustre and certain other species, the endodermis surrounds the entire stele. In E. giganteum, E. limosum and some other species, each vascular bundle is surrounded by separate endodermal layer. In still other cases, e.g., E. hyemale, E. ramosissimum etc., there is a common internal endodermis inside the ring of bundles delimiting the pith and common outer endodermis found outside the ring of bundles. Just beneath the endodermis a single layered pericycle is found. The pericycle is the outermost layer of the stele.

The vascular skeleton, i.e., stele of Equisetum is siphonostelic. The separated vascular bundles are arranged in a ring. The vascular bundles are situated opposite the ridges and alternate to the vallecular canals.

The number of bundles varies from species to species. In each internode, as many vascular bundles are found as there are leaves at the node. The vascular bundles alternate to each other in their position in the successive nodes. The internodal portions are longitudinally perforate, and each perforation runs the length of an internode. The perforations in the internode are situated above the traces. These perforations are not branch gaps. The vascular bundles are collateral type and resemble to some extent to that of monocotyledons. The vascular bundles contain both metaxylem and protoxylem. A carinal canal is developed in each bundle, because of the disintegration of the early-formed tracheids of the protoxylem during elongation of the surrounding cells of the internode. The remaining protoxylem elements are composed of few tracheids. These protoxylic elements are found arranged to the margin of carinal canal. The metaxylem elements are found in two groups. The two groups are found arranged on the margin of the carinal canal towards outside. The protoxylem lies in between the two groups of metaxylem. The metaxylem elements are composed of reticulate, scalariform or pitted tracheids. Sometimes spiral and annular tracheids are also found. The protoxylem is endarch and as supported by Eames (1909) and Miss Barrat (1920) the development of xylem is centrifugal. The phloem is composed of sieve tubes and phloem parenchyma. The sieve plates may also be seen. The companion cells are not found. The secondary growth is altogether absent. The central region of the stem is occupied by a hollow pith. The carinal canals are filled up with water.

Nodal anatomy: The alternate vascular bundles of the successive internodes are connected to each other by short branches and this way a continuous ring of the vascular cylinder is found in the node. Eames (1909) reported that the bundles at the nodes do not have carinal canals. Here, the protoxylem elements are intact and completely occupy the lacuna or carinal canal. At the node, the pith is not hollow and it forms a diaphragm separating the two successive internodes.

Rhizome: The anatomy of the rhizome is quite identical to that of the aerial shoot. However, the assimilatory tissue and the stomata are not found in the rhizome. The mechanical tissue, i.e., sclerenchyma is poorly developed as compared to that of the aerial shoot. In E. arvense, the pith is solid, whereas in certain other species, the pith and the vallecular canals are very much reduced.

Leaf: The anatomy of the leaf is quite simple. The leaves are uninerved, this means that each leaf contains a single vascular bundle. The vascular bundles of the leaf sheath are simple and collateral. The carinal canals are not found. Individual bundles are surrounded by separate endodermal layers. The outer tissues of the leaf sheath are composed of narrow sclerenchymatous bands. These bands of sclerenchyma pass up the leaf ridges and alternate with the strips of chlorophyllous tissue associated with stomata.

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