In this article we will discuss about the anatomy of equisetum with the help of diagrams.
Apical growth in rhizome and aerial stems is initiated by a pronounced pyramidal apical cell (Fig. 9. 1E) with three-cutting faces that are directed downwards. The apical cell cuts off cells in a continuous sequence to establish three tissue regions.
Stem anatomy of Equisetum has a rare combination of hydrophytic and xerophytic characters. The vascular system is characteristic of the plant, with variation sufficient enough to distinguish species. It is without a parallel in plant kingdom. Anatomically, the internodal region (Fig. 9.2A) differs from nodal region (Fig. 9.2F).
The outline of the internode is wavy with ridges and furrows (Fig. 9.2A). The epidermis is single-layered with stomata restricted to furrows (Fig. 9.2B), rhizome lacks stomata. The cortex is highly differentiated to suit the mechanical and metabolic needs of the plant. Beneath each ridge is a zone of sclerenchyma (Fig. 9.2A, B).
It is commonly restricted to peripheral region of cortex, as in E. giganteum, or may taper inwards (Fig. 9.2B). Also, at the base of the furrow is a small mass of sclerenchyma. Sclerenchyma cells are highly silicified.
The epidermal cells are also silicified and deposition of silica in epidermal cells is either in the form of discrete knobs or it covers the entire outer wall, giving the stem extremely rough texture.
Silica substitutes for low lignin content of plant and is significant for its erect texture. Silica is so essential for this plant that sporophytes developed in vitro from gametophytes formed viable spores only when the culture medium had at least 20 mg/L of silicon.
Adjacent to the ridges and below the furrows are areas of cortical chlorenchyma (Fig. 9.2B) with intercellular spaces, near the stomata. Since the leaves are small with few chloroplasts, cortical chlorenchyma is the chief photosynthetic tissue. Deeper in the cortex, corresponding to furrows or valleys and alternating with the ridges, are vallecular canals (Fig. 9.2B).
These are schizolysigenous canals extending the entire length of an internode. At the centre of the stem is the pith, which is hollow in well-developed aerial axes and is in the form of a diaphragm in very young internode. Occasionally, in the pith region of well-developed aerial axes can be seen a few disrupted parenchyma cells. Around the pith are vascular bundles in a ring.
Each vascular bundle is opposite the ridge and the number of bundles corresponds to the number of ridges, except in the extreme distal region. The cortex and stele are delimited by one or more endodermes. The number of endodermes is variable in different species and even in the same plant it may vary in different parts.
Three variations recorded are:
(a) A single endodennis surrounding the ring of bundles (rhizomes as well as aerial stem of E. arvense, aerial stem only of E. syluaticum),
(b) Two endodermes, one outside and one inside the ring of bundles (rhizome of E. sylvaticum and aerial stem of E. hyemale), and
(c) A separate endodermis around each vascular bundle (aerial stem and rhizome of E.fluviatile and rhizome of E. hyemale).
The vascular bundles of Equisetum are unique structures and there are contrasting viewpoints about their nature. In a very young bundle the protoxylem is towards the inner face. In a mature bundle, these elements get disorganized and lie on the periphery of protoxylem lacuna or carinal canal (Fig. 9.2E). In radial alignment with carinal canal is phloem; composed of phloem parenchyma and sieve tubes with sieve plates on diagonal walls.
Flanking the phloem and spatially separated from protoxylem are two groups of xylem elements. These xylem groups have been described as metaxylem. More appropriately, they should be described as lateral xylem as they have protoxylem elements and occasionally their own protoxylem lacuna, as in E. sylvaticum. In E. arvense are present tyloses on metaxylem.
A vascular bundle has been described to be endarch on the assumption that protoxylem elements represent a continuation of metaxylem (lateral xylem). On the contrary, studies based on young internodes have revealed that the bundle is mesarch and the evidence gets lost by the expansion of carinal canal. The protoxylem and metaxylem elements have annular and helical thickenings or have circular bordered pits.
Some species have true vessels. They are either simple perforation plates or are reticulate. These vessels are, however, restricted to internodal regions and do not form conducting channels as in flowering plants. The carinal canal is also conducting in function. It can be demonstrated, by placing an actively growing shoot, in a dye solution. The movement of dye is primarily through the canal.
The only mature tissues of the stem, traceable to apical meristem, are carinal xylem and protophloem. All other mature cells are the product of intercalary meristem. In a transection of stem passing through intercalary meristem. (Fig. 9.2F) is seen meristematic collenchyma which gives rise to cortical sclerenchyma. In it can be seen protophloem and protoxylem lacuna.
At a node, neither are vallecular canals nor carinal canals. The central region is not hollow and instead there is a pith diaphragm. The stele is a continuous cylinder of xylem (Fig. 9.2F) from which arise leaf traces and branch traces. At the level of leaf attachment there is trichotomy of protoxylem. The median bundle enters the leaf. The two laterals diverge right and left and meeting the adjacent lateral, joins with it to form vascular bundle of internode.
The leaf mesophyll is lacunate and there is a median mesarch vascular bundle. The cells of abaxial epidermis on their outer tangential walls have various types of ornamentation patterns. Stomata form longitudinal rows. On the adaxial surface of leaf, along the mid-vein region, are hydathodes.
The roots are slender and grow by means of an apical cell, the outermost derivatives of which form the root cap. Root stele has 2-6 protoxylem points surrounding a single central metaxylem element (Fig. 9.2G). In between protoxylem elements is the phloem.
The root in the genus has been described to have double endodermes but the cells of inner row are without casparian strips and therefore should be called pericycle. Root of E. ramossisimum unlike other species, bears paired branched root hairs.
Stoma in Equisetum is a highly specialized structure; both guard cells and accessory cells regulate its opening. In the formation of a stoma, a protoderm cell divides by two successive longitudinal walls into three cells. The two laterals are crescent-shaped accessory cells and the middle one the guard-mother-cell.
Following the production of guard cells, the accessory cells grow over and completely overarch the guard cells in such a way that they appear to be a result of periclinal division (Fig. 9.2C). On the lower surface of accessory cells develop a number of siliceous thickenings and they appear to radiate from stomatal pore (Fig. 9.2D).