Give a brief and illustrated account of the systematic position, habit, morphology and anatomy of the Equisetum.

Equisetum (horse tail), which is the only living genus of Equisetaceae, includes nearly 25 species.

It is world-wide in distribution, except Australia, New Zealands. Its various species grow in variety of habitats. Certain species grow in ponds and marshes, others grow in damp shaded places, and still others grow in exposed relatively dry habitats. E. arvense is a cosmopolitan species while E. debile is a common Indian species.

The plant body of Equisetum is sporophytic (2n) differentiable into leaf, root and stem, herbaceous, and perennial possessing a widely creeping much branched rhizome, from which arise erect and aerial branches, some are sterile and other fertile. The most common species E. debile attains a height of 10 to 15 and 0.5 to 100 cm. diameter. The largest known species. E. giganteum attains a height of 40′, while smallest known species E. scirpoides attained a height of 20 cm.. only.

Stem The stem of Equisetum is a underground rhizome, which is branched and differentiable into definite nodes and internodes. As each node is a whorl of scale which form a leaf sheath around the node. From the rhizome on the above side of the node there gives out the aerial erect branch and on lower side of node develop a tuft of adventitious root, which holds the soil. The aerial branches also possess the nodes and internodes. From the nodes of aerial branches possess whorls of small branchlets. All the branches and branchlets may also be differentiated into nodes and internodes, and on the nodes there remain present the whorl of scale leaves (Fig.)

The aerial branches which arise from the node of rhizome may be of the following three types:

(a) Sterile branches-richly branched, green

(b) Fertile branches-nongreen, unbranched.

(c) Intermediate branches-nongreen, unbranched, fertile in beginning but later becomes green and branched.

In E. debile there is no differentiation of sterile and fertile branches. E. arvense the sterile and fertile branches are well differentiable, while in E. palustre all the three types of branches remain present.

Leaves: The leaves are simple, small, uninerved, slender and scale like. They are arranged in whorls on the nodes. They form a brown sheath enveloping the base of internode with teeth like projection at the apex. The number of leaves are usually devoid of chlorophyll and are entirely protective in function.

Root: The roots are adventitious develop from the node of rhizome. The roots do the function of fixation of plant and absorption of water and mineral salts from the soil. Sometimes the root also arise from aerial branches but they are non-functional.

Stem: The stem anatomy is slightly different in different parts i.e. internodal region, nodal region, sterile branch, fertile branch and rhizome, although the general organization of the stem is same in different parts.

Internode of Aerial Sterile Branch: A transverse section of the internode of sterile branch is weavy in out line having ridges and grooves. It is organized into epidermis, cortex and stele. The epidermis is outermost layer with few stomata in the grooves. The cells of epidermis are remarkable for their strongly silicified outer walls. Below epidermis their remain present in the cortex. The cortex contains a variety of tissues. On the periphery below the ridges, are longitudinal ribs of sclerenchyma which gives mechanical strength to the stem. Alternating with these sclerenchyma bands, below grooves are the bands of assimilating tissue containing chlorophyll-the chlorenchyma, which take the place physiologically of the much reduced leaves. Below at the inner parenchymatous, cortex remain present continuously and have large innercellular cavities below each grooves-called as the vallecular canal, forming no doubt, an aerating system. The cortex is bounded on the inside by an endodermis and with in it lies the pericycle forming the external layer of stele. The stele in this case is siphonostelic. The vascular tissue as consists of ring of vascular bundles and a pith hence called as eustele or polyfascicular siphonostele.

The vascular bundles in the ring are widely separated by broad bands of parenchyma. Each of the vascular bundle remain present below each ridge. Each bundle is collateral, that is, it has the xylem on the inner and phloem on the outer side. The xylem has a shape of V, with endarch protoxylem. Presence of inter cellular canal, the carinal canal at the base of vascular bundle is a remarkable feature. The phloem lies between the two limits of the xylem. The centre of the stem in young stage is made up of parenchymatous pith, as the stem becomes mature the cell dissolves and form a hollow central cavity.

A transverse section of the internode of fertile branch closely resembles to the internodes of sterile branch in anatomical characters except of the following differences :

(1) Absence of chlorenchyma. (2) Absence of stomata.

A transverse section of the internode of rhizome resembles to the internode

of sterile branch in anatomical characters except the following differences :

1. Absence of ridges and grooves, 2. Absence of sclerenchyma and chlorenchyma, 3. The pith is solid parenchymatous, 4. Absence of sclerenchyma.

The anatomical character of the nodes are similar to that of the internode except the following differences :

1. Absence of carinal canals. 2. Pith is solid. 3. Absence of vallecular canal.

Root : A transverse section of root shows a wide cortex, with an outer exodermis (Fig.). Inside there is a well defined endodermis and a small stele consisting of either three or four protoxylem groups, with phloem alternating with them and a central metaxylem. The chief interest, in the root of equisetum is fact that the endodermis consists two layers, a large called-outer and smaller called inner endodermis.

Leaf : The leaf of Equisetum consists of a single collateral vascular bundle. Single called endodermis and mesophyll. The mesophyll possesses alternating strips of assimilatory tissue and narrow bands of sclerenchyma.

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