Give an illustrated account of the life history of Equisetum.

Ans. Reproduction or Spore producing organs: The adult plant of Equisetum represents the sporophytic phase of life history. It reproduces by means of spores which are formed in a special sac like structures called Sporangia. Sporangia are usually borne on the hexagonal peltate structure called as sporangiophore. Several sporangiophore are arranged in compact structure called cone or strobillus.

The cone of Equisetum is a terminal structure, arising either on the main fertile shoot or rarely, on a branch. It possesses a fairly stout axis, giving rise to crowded, alternating whorls of ‘T’ shaped peltate scales, termed sporangiophores on which the sporangia are developed. Each sporangiophore has a short cylindrical stalk and expands at its distal end into a flat plate, on the underside of which five to ten sporangia are attached. At the base of the cone there is ring like out growth the annulus.

Morphologically the strobilus is supposed as a shortened axis possessing nodes and internodes and sporangiophore as a lateral branch possessing a whorl of sporangia or fertile ventral lobe of dorsiventral sporophyll, whose dorsal lobe is lost.

The Sporangium when mature is an elongated cylindrical sac like structure with a rounded apex. It consists of a jacket or a single layer of elongated cells containing the ripe spores which are all alike (homosporous).The cells of jacket are marked with spirally thickened band upon dorsal surface and annular ones on the ventral side of the sporangium.

The development of sporangium takes place from the single superficial cell around the crown of the developing sporangiophore. Growth in the centre of the sporangiophore tip soon pushes the sporangium initial laterally and ‘over the edge’ so that they become inverted with reference to their earlier position. The sporangium initial divides into a outer and inner cells forming a outer jacket and inner sporogenous tissue by repeated divisions. The jacket layer divides to form the spore mother cells (2N) or sporocytes. The spore mother cells divides by reduction division (Meiosis) and followed by a simple division resulting in the formation of the spores tetrads (n). The spores (haploid) get separated from each other and all are alike i.e., homosporous.

Spores are the units of gametophytes. The spores are homosporous. Spores: The spores are haploid in nature (n) and on germination gives rise to the young prothallus or gametophyte.

The young spores when first formed, has a thin cellulose wall but as it develops it becomes more complicated. The wall of the spore becomes four layered viz. Perispore colourless outermost layer, middle layer-cuticular, exinelight bluish and intine-innermost yellow. The outermost layer or perispore splits along spiral lines into two long band with flattened ends, called elaters, which until the spore is mature, remains clearly stretch themselves out, crosswide, remaining attached only by their central region and forming four distinct appendages. Each spore is uninucleate and contains chloroplasts. All spores are like i.e. homosporous. The functions of elaters are as follows:

1. Elaters help in splitting of sporangium.

2. Being hygroscopic in nature helps in dehiscence and dispersal of spores.

3. Due to entangled nature, several spores are liberated at a time.

The spores with elaters are present in the Sporangium. When the conditions are dry due to hygroscopic nature of elaters the spores get liberated free by the substance of sporangium wall.

The spores (along with elaters) get liberated free from the sporangium and taken away by wind here and there. The spores of Equisetum remain viable only for a few days after liberation. The spores under natural conditions are generally found growing upon clay soil and in the mud belt along the banks of streams, rivers etc.

The spore, which is the unit of gametophyte on getting a suitable substratum withdraws its elaters and get attached to the substratum. Under favourable conditions the spore germinates radially and usually within 10 or 12 hours after the spore is sown, the germination of spore starts in gametophyte.

The spore first of all divide by a transverse division to form a lower smaller rhizoidal cell and an upper, bigger cell. The lower cell loses its chloroplasts, elongates forming the first rhizoid which is positively geotropic. The larger upper cell containing chloroplasts develops into rest of the gametophyte.

In few cases when the conditions are unfavourable the upper bigger cell elongates; and divides by the several transverse divisions, so that a longer or shorter filament is formed, it represent the gametophyte of equisetum, this is normally non-viable.

In most of the cases, when the conditions are favourable the upper higher cell elongates; and the division and redivisions occur irregularly resulting in the formation of a small cushion like tissue, several cells in thickness and heating rhizoids on its lower surface. The meristematic regions get developed on the upper portion of causion, which continues the growth and give rise to additional upright green branches; known as photosynthetic filament.

Thus, the mature prothallus or gametophytes of Equisetum is a cushion like multicellular structure which shows two cell marked region i.e., compact cushion like basal region and the vertical erect green photosynthetic lobes. The gametophytes are long lied independent structures. On the lower side of gametophyte the rhizoids remain present which do the function of absorption and fixation.

If the conditions of growth are favourable, the first sex organs are formed when the gametophytes are 30 to 40 days old. The initials developing into sex organs are in the meristematic region of the cushion. The mature sex organs lie between plates, somewhat form the margin of a thallus because the marginal meristem continues marginal growth and a formation of vertical plates.

Whether Equisetum’s gametophyte is monoecious or dioecious is a controversial question. Many experimental works have done on this problem and the study of Kashyap on E. debile, Walker on E. laevigatum, Castle on E. arvense and the observation of all these shows the gametophytes of Equisetum are usually monoecious, bisexual and under certain conditions shows partial dioecism and the semisegregation of sexes can be ascribed to the starved conditions of the plant. The archegonium develops first and antheridia afterwards.

Any superficial of the marginal meristem (photosynthetic region) function as archegonial initial cell. It divides by a periclinal devising forming an outer primary cover cell and inner central cell. The primary cover cell divides quadrately to form four primary neck cells. Transverse division of these cells results in an archegonial neck which projects vertically above the thallus. The central cell divides transversely into a primary neck canal cell and a primary venter cell. The venter cell further divides into two by a transverse division forming a venter canal cell and an egg. In some species the neck canal cell remain single where in other species divides vertically into two boat shaped neck canal cells.

Thus the mature archegonium has a base sunken it the prothallus and a projecting neck. The neck is short and consists of four vertical rows and cells. It has 2 neck canal cells, one ventral canal cell and egg.

The antheridium usually develops after the development of archegonium from the meristem of gametophyte. The antheridia may be either on the margins or on the apex delicate lobes. There are usually of two types of antheridia-the embedded type and the projecting type. The embedded type generally develops on the massive parts of the normal gametophytes and the projecting type is usually formed on the margins of the apex of the delicate lobes.

During the development of projecting or embedded type of antheridium any superficial cell of massive region or filamentous region functions as antheridial initial cell jacket and inner primary androgonial cell in the case of embedded type while in case of projecting type the antheridial initial cell divides by three periclinal divisions so as to form jacket cells and one central androgonial cell. Now in both the cases the jacket initial forms by further divisions forms the jacket while the androgonial cells divides and redivides to form the androcytes. Each of the antherozoid get metamorphosed into antherozoid which is spirally coiled and flattened throughout the greater part of its body. The anterior end is rounded and bears numerous flagella.

Fertilization takes place in the presence of moisture. The neck canal cells and venter canals cells get disorganised and produce a chemical liquid of the nature of malic acid. Antherozoids or sperms get attracted by this chemical liquid and many antherozoids enter the neck of archegonium-but only one of them penetrates the egg-resulting in the formation of zygote which has a diploid (2X) nucleus. Unlike other pteridophytes, usually more than one archegonium is fertilized and several (8 to 14) sporophytes may develop on one prothallus.

The zygote is the first cell or unit of the sporophytic generation.

The zygote or oospore starts its germination while embedded in the gametophyte. It increases in size and divides by a transverse division into an upper epibasal and lower hypobasal cell. The first division of zygote followed by a vertical division resulting in the formation of quadrant embryo. The upper epibasal cell forms the stem and leaf, while the lower hypobasal cell develops into root and foot. There is no suspensor formation in Equisetum. The stem portion of embryo enlarges further and comes out the neck of archegonium. The neck of archegonium forms the calyptra. The primary stem grows straight and gets differentiated into nodes and internodes. Each node bears three leaf teeth. A series of branches arise from the base of primary stem to form the underground rhizome. The primary root also wither and a number of adventitious root arise at its place.

Graphical representation of the life cycle of Equisetum showing alternation of generation :

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