In this article we will discuss about the process of reproduction in equisetum with the help of diagrams.
The cones (Fig. 9.3A) are terminal either on the main shoot as in E. arvense (Fig. 9.1 A, B), or on smaller lateral branches as in E. heymale. At the base of cone is a ring-like structure, the annulus or collar, formed by the complete fusion of sterile appendicular structures. Occasionally, and especially in E. arvense the axis continues to grow and forms indeterminate structure.
In some species is shoot dimorphism (sterile and fertile shoots). Vegetative shoots are green and branched and fertile cone-bearing-shoots are brown and unbranched in E. arvense and E. sylvaticum. In the latter the fertile shoots turn green on shedding of spores, green branches grow from nodes of brown unbranched shoot.
The cones are made up of compactly arranged whorls of peltate appendages, the sporangiophores. At successive whorls the sporangiophores alternate but somewhat irregularly, due to condensation of axis. Each sporangiophore (Fig. 9.3B) has a slender stalk by which it is attached to the strobilus axis and its free distal end is expanded into an hexagonal disc.
The discs of sporangiophores at the basal and apical regions are of different configurations to suit the shape of cone (Fig. 9.3A). On the underside of the sporangiophore disc (Fig. 9.3B) are borne 5-10 elongate sporangia with their round apices facing the central axis. The sporangiophores have been variously interpreted as sporophylls, stems or other organs.
In the cone axis, there is greater development of internodal xylem with relatively less xylem at the nodes resulting in a netted appearance, and without large canals as seen in stem. The vascular supply to the sporangiophore arises from the nodal ring of the cone axis and after traversing up to the end of stalk, it dichotomizes to supply one trace to each sporangium.
The primordia of sporangiophores are dome-shaped (Fig. 9.3C) and develop at the apex in position where leaf primordia are formed, on vegetative apices. Sporangial initials differentiate on the dome of sporangiophore apex very early, and later with the expansion of the dome the sporangia are pushed to the underside of the disc.
Sporangia are eusporangiate. However, all sporogenous tissue can be traced to a single initial. Therefore, it has been questioned whether the sporangia originate from one cell or a group of cells.
The jacket of a developing sporangium is several cells in thickness, the inner layers of which function as tapetum. In a mature sporangium the jacket is only two cell layers. Cells of the outer layer have spirally thickened walls. As the sporocytes are formed the walls of tapetal cells breakdown and their contents fuse forming periplasmodium.
Before reduction division, about one third of the sporocytes also disintegrate and their contents get mixed up with that of tapetal cells forming a plasmodial liquid. The developing spores soon become spherical and come to have very elaborate wall structure, mainly due to the activity of periplasmodium.
As the spores mature, four acellular strap-shaped bands with spoon-like tips are deposited on their outer surface. These bands are attached at a common point and are hygroscopic, remian coiled to the spore when moist and uncoil as the spores dry. These bands are referred as elaters. Their function is, however, uncertain.
Possibly, their expansion assists the sporangium in dehiscence or their contraction and expansion help in long distance dispersal of spores. The spores are large, green with a brief viability period ranging from one to forty-eight hours after dispersal. Germination on a moist substratum is possible within two days.
First division of a germinating spore is unequal (Fig. 9.4A) and takes place within ten to twelve hours forming smaller rhizodial cell and larger prothallial cell. From the latter, by a series of unequal divisions, is formed the prothallus (Fig. 9.4B). The mature prothallus has a basal pad with upright lobes (Fig. 9.4C).
The rhizoids are borne on the undersurface of the pad. Gametophytes are green pin-head cushion-like structures, often indistinguishable from moss protonema or algal growth on the river banks, but in tropical species they are compact cushion-like structures, of up to 3 cm diameter and when growing on exposed sand turn bright red.
Sex determination in Equisetum is influenced by environmental conditions. Incipient to complete heterothallism occurs in the genus, without accompanying heterospory. Among Latin American horse-tails, E. giganteum is bisexual, the gametophytes produce antheridia and archegonia simultaneously. In contrast, gametophytes of E. myriochaetum are either male or female.
The latter, in absence of fertilization, develops antheridial lobes. The gametophytes of E. bogotense are unisexual and do not change sex. Thus there is a series illustrating sexuality from bisexual condition to a unisexual condition through a transitional stage of plasticity.
The archegonia (Fig. 9.4D) are confined to cushion region, in between the aerial lobes (Fig. 9.4C), and project by their 3 or 4-celled high necks. The axial row consists of an egg, venter canal cell, and one neck canal cell. In some species there are two neck canal cells which are boot-shaped and lie side by side.
The antheridia are embedded and elongate (Fig. 9.4E) structures normally present on the aerial lobes but rarely also occur in the cushion region. They are massive and produce large number of spirally coiled, multiflagellate (Fig. 9.4F) antherozoids. Each sperm has about 120 flagella.
The embryogeny in Equisetum has been poorly documented. The first division of zygote is transverse (Fig. 9.4G). No suspensor is produced and the embryo is exoscopic. On the apical region is established the stem apical cell (Fig. 9.4H). First leaves are either embryonic in origin or may form from stem apex. The root may arise from entire lower embryonic hemisphere or only from one side and the other side forms the foot.