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The plant body of equisetum is a sporophyte and reproduces by means of spores. The spores are found in sporangia, which develop on the underside of the peltate shield-shaped hexagonal sporangiophores. Cluster of whorled sporangiophores are arranged in compact cones or strobili and develop at the tip of the branches or main stems. In some cases, all the stems produce terminal strobili. These fertile branches appear in the spring and are somewhat yellowish in colour. They differ from species to species.

The strobilus or the cone is situated at the apex of the fertile aerial branch. It consists of a thick and stout central axis on which the whorls of densely crowded peltate appendages are arranged. The appendages are the sporangiophores which bear sporangia. Each whorl contains many sporangiophores.

Each sporangiophore consists of a short cylindrical stalk which expands as a flattened peltate disc at right angles to the central axis of the strobilus. The peltate disc is hexagonal in outline and bears sac like sporangia on lower surface. The sporangia are 5 to 10 in number and hanging vertically at right angles to the peltate disc towards the axis of the strobilus.

A single superficial cell around the row of developing sporangiophore near the margin gives rise to all the essential parts of the sporangium, though the adjacent tissues also take part in the further growth of sporangium. Active growth in the centre of sporangiophore pushes the sporangial initials laterally and over the edge, so that they become inverted and come to point with their apices towards the axis. The sporangial initial divides by periclinal wall into an outer cell and an inner cell.

In majority of the species, the reproductive structures develop at the apices of the branches. These reproductive structures are called strobili (singular strobilus) or spikes. The strobilus is a compact structure of sporophylls. The sporophylls are quite similar in structure to that of ordinary vegetative leaves. Each sporophyll bears a ligule at its base. Each sporophyll bears a single stalked sporangium. The sporangia are of two types i.e. microsporangia and megasporangia. The sporophyll which bears microsporangium is microsporophyll, and which bears megasporangium is megasporophyll. Both micro and megasporophylls may be found in same strobilus or in different strobili. The sporophylls are always spirally arranged on the axis of the strobilus but in a very much condensed form. In majority of the species the micro and megasporophylls borne in the same spike, e, g., S. kraussiana. According to Mitchell (1910), in certain species e. g. S. gracilis and S. atroviridis the strobili bear only one type of sporophylls. When the strobilus bears both kinds of sporophylls, the megasporophylls may be found in the lower portion of the strobilus and the microsporophylls in the upper portion of the strobilar axis, e. g., S. spinosa and S. rupestris. In certain other species the mega and microsporophylls may be arranged in different vertical rows in the strobilus.

The interpretation of the plant body as a branched axis with its divisions specialized for various functions, but all like in fundamental structure, is called the Telome theory. The Telome theory is due to the work of Zimmermann. He goes back to the earliest and simplest type of vascular plant known, that in the Psilophytales in which the body consists of branching leafless axes, which are all substantially alike. In some cases these may be arranged dichotomously, in others monopodially, and the difference may have arisen by “overtopping”, but in either case these naked axes as the primitive units of construction of the cormophytes, which is thus regarded as primarily a system of axes. Each of the terminal branches in such a system is called a telome. Each telome is an ultimate branch on an older axis or mesome. Some telomes are sterile, others bear terminal sporangia and are therefore fertile. Telomes also tend to unite into groups, called syntelomes, which may be all sterile, all fertile or mixed. According to Telome theory the earliest leaves are flattened telomes or phylloids, and the sporangium in all cases, together with its stalk or sporangiophore, is a fertile telome. Compound sporangiophores, like those of Equisetum, are syntelomes. The phylloids are the prototypes of the microphyllous foliage leaves. In the most primitive condition the sterile and fertile telomes were apparently quite independent of each other. To axillary position which the sporangial telomes occupy in Lycopsida has been arrived at by secondary shifting of the telomes and shows three ways in which the change may have taken place.

The elongated or oval sporangia were terminally developed at the apices of the branches of the aerial shoot. It is not clear whether the sporangia were borne on all the apices of the branches or on certain branches of the plant. Each apex bore a single sporangium.

As regards the shape, the sporangia were cylindrical and they possessed the greater diameter than those tips of the branches on which they were found. The jacket layer of the sporangium consists of several cells in thickness. The outermost cells of this layer were thick-walled and cutinized at their outer face. The middle layer or the sporangial wall possessed thin walled cells and inner most layer consists of thin walled rounded cells which probably act as tapetal layer. The sporangium had no columella with in it. The cavity contained many spores of the same size. The sporangia were indehiscent and it is thought that the spores were released only on the decay and disintegration of the thick sporangial wall.

The spores are spherical and cuticularized. They are found arranged in spore tetrads.

A stele in which the vascular cylinder consists of a solid core of xylem surrounded by phloem, pericycle and endodermis was named as protostele by Jeffrey (1898). There is no pith in a protostele. It is a fundamental type of stele of vascular plants. In most of the pteridophytes, the stem remains protostelic at sporeling stage.

Brebner (1902), Worsdell (1902), Zimmermann (1930) and others have categorized the protosteles into the following types :

1. Haplostele: The “protostele with a smooth core of xylem” (Smith, 1955) surrounded by a uniform layer of phloem was named as haplostele by Brebner (1902). It has been observed in fossil genera like Rhynia, Horneophyton, and living genera like Selaginella chrysocaulos, S. kraussiana, S. selaginoides, S. willdenowii, Gleichenia dichotoma, Lygodium, and Cheiropleuria.

2. Actinostele: It is a protostele having a xylem core with radiating ribs. In this case the xylem core is star shaped or stellate. Brebner (1902) named such a protostele as actinostele. In actinosteles the phloem is not present in a continuous manner but in the form of separate groups which alternate with the distant ends of the star shaped xylem, e.g., Asteroxylon, Psilotum, Lycopodium serratum, and Sphenophyllum.

Because of the breaking up of the xylem mass into different forms, the actinosteles show the following variations or types:

(i) Plectostele: In some cases like Lycopodium clavatum and L. volubile the xylem gets broken into a number of more or less parallel plates. Such xylem plates alternate with the phloem plates. Zimmermann (1930) named such an actinostele as plectostele.

(ii) Mixed Protostele: In this type the xylem groups are uniformly scattered in the ground mass of the phloem. Such actinostele is called mixed protostele as in Lycopodium cernuum.

(iii) Mixed Protostele with Pith : Sometimes, thin parenchyma cells remain associated with the xylem in the mixed protostele. In genera like Hymenophyllum demissum, H. dilatatum, Lepidodendron intermedium and L. selaginoides the centre of the protostele is occupied by the parenchyma cells among the tracheids. Such parenchyma cells represent the mixed pith and such a type of stele is called mixed protostele with pith. The genera having such a type of stelar organization form a link between protostele and siphonostele because of the presence of parenchyma cells in the form of pith.

The origin of the sporophyte in land plants represents a fundamental phase in the plant evolution. There are two theories concerning the origin of the alternating generations in land plants: the ‘Antithetic’ and the ‘Homologous’ theory. These have never been fully resolved.

The homologous theory states that a mass of cells forming mitotically from the zygote adopted the same developmental plan of the gametophyte, but giving origin to a diploid sporophyte.

Considerable evidences at morphological, cytological, ultrastructural, biochemical and especially, molecular level, strongly suggest that the land plants evolved from green algal ancestor, similar to those belonging to the genus Coleochaete, Chara and Nitella living today. Their organism is haploid for most of their life cycle, and diploid only in the zygote phase. On the contrary, the land plants are characterized by a diplo-haplontic life cycle. Several questions are implied in these theories, and numerous problems remain to be solved, such as, for example, the morphological difference between gametophyte and sporophyte and the strong gap existing between these last with a sporophyte dependent on the gametophyte and the pteridophytes having the gametophyte and sporophyte generations independent.

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