Equisetum, commonly known as ‘horsetail’ is found with about 25 species all over the world excepting Australia and New Zealand, and is mostly an inhabitant of the cool and temperate regions. It is usually found in hills, especially in dark and marshy places, wet fields, gravelly or loamy soil. The common Indian species of Equisetum are E. diffusum, E. debile and E. robustum.
The Sporophyte of Equisetum:
The plant is a herbaceous perennial one, but many are evergreen The size of the plant is very variable, usually not exceeding a metre in length, but E. giganteum, a South American tropical species, grows to a height of about 10-20 metres.
The stem is a much-branched, dorsiventral, horizontal, rhizome which gives off many erect sub-aerial shoots, usually of two distinct types, sterile and fertile. The sterile shoots are green in colour usually branched, perennial and vegetative in function. The fertile shoots are pale or brownish in colour, without chlorophyll, un-branched, short-lived and wither after discharging their function (the production of spores).
Transitional forms between these two types are not uncommon. The stem is jointed with nodes and internodes, and each internode consists of distinct ridges and furrows, the number of ridges is equal to the number of leaves present at each node. At each node there is a whorl of minute scale leaves, which are mostly non-green and soon become dead.
These are free at the tips but fused laterally with one another forming a sheath closely enveloping the base of each internode. The function of photosynthesis is, however, taken by the stem and branches. The branches are in whorls and usually equal in number to the leaves, a branch always developing between a pair of leaves. The whorls of leaves and branches of successive nodes alternate with one another.
Roots are slender, fibrous, and adventitious in nature and arise endogenously from the bases of the lateral branches or dormant buds of the rhizome.
A cross-section of the stem shows two regions clearly differentiated, the cortex and the stele. The cortex is bounded on the outside by an epidermis which is single-layered and provided with a number of alternate ridges and furrows. The outer walls of the epidermal cells are highly silicified. Stomata are present in two vertical rows in each furrow.
The cortex is highly differentiated into the following:
(a) Hypodermis consisting of sclerenchyma which may be restricted to the periphery of the cortex or may extend inwards to the stele; below each ridge sclerenchyma is well-developed.
(b) General cortex consisting of many-layered parenchyma with large air cavities (vallecular cavities) , each corresponding to a furrow, and these alternate in position with usually water-filled cavities (carinal cavities) in the vascular bundles The peripheral portion of the general cortex, lateral to the sclerenchyma, is chlorenchymatous with intercellular spaces and especially conspicuous just beneath the stomata, and
(c) Endodermis surrounding the entire stele with prominent casparian strips.
In some species, each vascular bundle is surrounded by an individual endodermal layer. Lying internal to the endodermis there is the pericycle consisting of a single layer of parenchyma cells. The vascular bundles are closed, collateral, and conjoint being arranged in a ring and each containing a carinal cavity; they are laterally separated from one another by a small mass of parenchyma cells.
The protoxylem is endarch and in mature bundles these are found in a more or less disorganized condition within the carinal cavities. Outside the carinal cavity there is a mass of phloem consisting of phloem parenchyma and sieve tubes. On both sides of the phloem mass is a small metaxylem mass consisting of tracheids only.
The pith lies at the centre. Occupying most of the pith there is a conspicuous space (the central cavity) in the internodes of the primary branches of the aerial shoots, and this central cavity may be entirely absent in the internodes of smaller branches and of the rhizome; these cavities also are usually water-filled.
Internally, the root of Equisetum possesses an exodermis followed by a thick cortex. There are two endodermal layers —the outer endodermis and the inner endodermis, which replaces the pericycle. There are usually four protoxylem groups alternating with the phloem and a big central metaxylem.
At the apex of the fertile branch there is a cone-like structure, the sporangiferous spike or cone or strobilus, which consists of a central axis bearing a variable number of sporangium-bearing organs, the sporangiophores, in distinct but compact whorls.
Each sporangiophore is a peltate structure with a somewhat flattened hexagonal disc and is attached to the main axis at right angles to each other. When young, these sporangiophores remain closely fitted together, but later on separate as distinct whorls. At the base of the strobilus, in some cases, a modified leaf-whorl is present forming the so-called annulus.
On the under surface of each sporangiophore, towards the circumference, sporangia, usually 5-10 in number, are borne on a ring and are projected horizontally towards the axis of the cone. Each sporangium is an elongated, sac-like body containing many spore mother cells, from each of which four spores are formed due to reduction division, so that a mature sporangium contains many spores inside. With reduction division and formation of spores, the gametophytic or haploid generation begins.
The Gametophyte of Equisetum:
The sporangium, at maturity, bursts longitudinally and sets free a large number of green-coloured spores. Each spore, besides its usual two coats, has a delicate cuticular layer and a thick perispore. Later on, this last layer splits into four ribbon-like appendages called elaters, with spoon-like tips and these remain attached to a common point.
These elaters are hygroscopic in nature, which uncoil in dry air and recoil around the spore in moisture. The function of the elaters is uncertain. Possibly, their expansion may assist in the dehiscence of the sporangia, or possibly their expansion and contraction may assist in spore-dispersal. The spores are all of one kind. Hence, Equisetum is homosporous.
On germination, each spore forms a small green prothallus which differs in form and in the position of sex organs (antheridia and archegonia) from that of a fern. At maturity, it looks like a disc-shaped thick cushion of tissue consisting of several layers of cells, from the upper surface of which there arise irregularly-lobed flattened branches, each one cell in thickness. It produces rhizoids from its lower surface.
Antheridia usually develop at the upper ends of these flattened and vertical branches, while the archegonia are produced on the upper surface of cushion near the bases of these branches. Thus, the prothallus is monoecious. On the other hand, if the gametophytes remain small, each one-cell in thickness, probably due to nutritional deficiency, they bear antheridia only and it is for these reasons they were once thought to be dioecious.
In some cases (E. arvense), half of the spores develop male gametophytes and the other half, female gametophytes under controlled conditions. In the latter case, if fertilization of the archegonia fails prothallia proceed to develop antheridia. The prothallia may be long-lived and may persist for more than two years.
Each antheridium consists of an outer jacket layer, one-cell in thickness, surrounding a number of spermatozoid mother cells (spermatocytes), each of which is ultimately metamorphosed into a spirally-coiled antherozoid with numerous flagella.
The archegonium consists of a neck (3 or 4 cells in height) and a central cell projecting vertically above the prothallus. The central cell divides transversely into a primary canal cell and a primary ventral cell. The primary canal cell vertically divides into two neck canal cells, while the ventral cell asymmetrically divides into a ventral canal cell and an egg. At maturity the canal cells disintegrate and the egg is ready for fertilization.
Fertilization is effected by antherozoids, which swim down the canal of the archegonium and only one fuses with the egg. The fertilized egg soon surrounds itself with a wall and forms an oospore. With fertilization and, formation of oospore, the sporophytic or diploid generation begins.
The New Sporophyte of Equisetum:
The oospore passes into an embryo (consisting of stem, root, cotyledon, and foot) which, without any period of rest, continues to grow until the Equisetum plant is established and thus completes the life cycle. It has been observed in the case of E. debile that a single gametophyte may bear eight to ten young sporophytes.
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