Pteridophytes are vascular cryptogams. Plants are small sized annuals or Perennials which reach a longer size and lie in damp, shady places. Plants have large leaves and small stems and are known as megaphyllous e.g.: Fern. On the other hand plants have smaller leaves and larger stem and are called microphyllous e.g.: Selaginella, Equisetum. Plant itself is sporophytic in nature.
Life cycle of Equisetum:
Plant body of Equisetum is sporophytic and shows ell defined alternation of generation in life cycle. Sporophytic plant body produces spores which give rise to gametophytic prothallus. Which undergoes gametic fusion again to produce zygote which is sporophytic in nature which by embryo develops into young sporophyte.
Sporophytic phase:
Reproduction by spore production organs: Adult plant of Equisetum bears sporophytic phase of life cycle. It produces by means of spores formed in special sac like sporangia borne on under side of hexagonal pellate structure called sporangiophore. They are arranged in compact structures called come or strobillus. Cone of equisetum is terminal structure arising either on main fertile short or rarely on a branch. It has stout axis giving rise to crowded alternating whorls of T shaped petate scabs termed sporangiophores in which 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 cone is ring like out growth the anmulus. Sporangium when mature is an elongate sac like with a rounded apex. It has jacket or single layer of elongated cells with ripe spores which are all alike i.e. homosporous.
Development of sporangium takes place from single superficial cell around the crown of developing sporangiophore. Sporangium initial divides into outer and inner cell to form outer jacket and inner sporogenous tissue. Jacket layer divides to form spore mother cells or sporocytes. Spore mother cells divide by reduction division (Meiosis), which follow simple division and form spores tetrads. The spores (haploid) get separated from each outer and all are alike or homosporous.
Gametophytic phase:
Spores are the units of gametophytes. Spores are homosporous or haploid in nature and on germination give rise to young prothallus or gametophyte. Wall of spore becomes four layered i.e. Perispore colourless out most layer, middle layer cuticular, exine light bluish and intine inner most yellowish. Perospme splits into two long bands with flattened ends called elaters. Each spore is unimucleate and contains chloroplasts. All spores are alike i.e. homosporous.
Elaters help in splitting sporangium and helps in dispersal of spores.
Spores with elaters are present in sporangium. When conditions are dry due to hygroscopic nature of elaters the spores get liberated free by the rupture of sporangium well.
Germination of spore and formation of gametophyte:
The spores (along with elaters) get liberated free from the sporangium and take away be wind here and there. The spore which is the unit of gametophyte on substratum withdraws its elaters and get attached to substratum.
Under favourable conditions the spore germinates and starts gametophyte.
Spore divides to form lower smaller rhizoidal cell and upper bigger cell. Lower cell loses its chloroplasts elongates to form rhizoid which geotropic. The larger upper cell containing chloroplasts develops into rest of gametophyte.
Mature prothallus or gametophyte of equisetum bears cushion like basal region and ventral erect green photosynthetic lobes. On lower side of gametophyte rhizoids are present which do the function of absorption and fixation.
Structure and development of sex organs on gametophyte:
In favourable conditions sex organs are formed. Mature sex organs lie in the margin of thallus when gametophytes are bisexual development of archegonium takes place first and of antheridium afterwards.
Archegonium: Any superficial cell of the marginal meristem functions as archegonial initial cell. It divides by periclinal division to form outer primary cover cell and inner central cell. Primary cover cells divide to form four primary neck cells which project above protahllus. Central cell divides into a primary neck canal cell and primary venter cell, which further divides into two to form venter canal cell and an egg. Mature archegonium has base sunken in the prothallus and a projecting neck which is short with four vertical rows and cells. It has two canal cells one venter canal cell and an egg.
Antheridium:
Antheridium develops after archegonium from meristem of gametophyte. Antheridia may be either on margins or on the apex delicate lobes. Antheridia are of two types the embedded type and projecting type. Embedded type develops on massive parts of normal gametophytes and projecting types is formed on margins of apex of delicate lobes.
During development any superficial cell of massive region or filamentous region functions as antheridial initial cell, jacket initial and inner primary androgonial cell in case of embedded type. While in projecting type antheridial initial cell divides by three periclinal divisions to form jacket cells and one central androgonial cell. Androgonial cells divide to form androcytes. Each antherozoid gets metamorphosed into antherozoid which is spirally coiled. The anterior end is rounded and bears many flagella.
Fertilization: It takes place in moisture. Antherozoids or sperms get attracted and enter the neck of archegonium but only one penetrates the egg, fuses with it to form zygote with haploid (2X) nucleus. 8 to 14 sporophytes may develop on one prothallus. Zygote is the first cell or unit of sporophyte generation.
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