Selaginella plant is sporophytic in nature. Selaginella has two distinct phases the sporophytic and gametophytic which alternate with one another. Plant bears root stem and leaf.
Aeterospory:- Selaginella plant after reduction division in sporangium. Selaginella is heteroporous germs, it produces two distinct types of spores, microspores (smaller) and megaspores (bigger). Micro and megaspores are produced on microsporangium and megasporangium which are stalked and present on microsporophylls and megasporophylls. These sporophyll together form compact mass called strobillus.
Sporophytic Phase:- Mature plant of selaginella is sporophyte which bears leaf, stem and root and has strobillus at apices of branches. Strobillus possesses many ligulate sporophylls each bearing on upper side and near its base small short stalked sporangium. One type of sporangia contains four large pores and others contain many small spores. Large spores are megaspores, sporangium bearing them is megasporangium and leaf on which borne is megasporophyll. Small spores are microspores, sporangium bearing them microsporangium and leaf microsporophyll. Such phenomenon of dimorphism of spores is called heterospory.
Development of sporangium:- Sporangium develops from upper surface of sporophyll. One or more cells from upper surface of sporophyll function as sporangium initial cells which divide by preclinical division into two, forming outer jacket initials and inner archesporial cells which by many divisions form mass of sporogenus tissue. Cells in outer layer of jacket become thick walled. There is formed differentiation of tapetum. Sporogenous tissue gets separated from each other to form spore mother cells or sporocytes.
Development of microsporangium and microspores:
In micro-sporangia 80 to 90 % spore mother cells remain functional. Reduction division takes place in each spore mother cell followed by simple division (Mitosis) whish results in the function of four microspores from each microspore mother cell.
Development of Mega-sporangium and Megaspores:
In case of mega-sporangium all the spore mother cells except one, get disintegrate. Only megaspore mother cell develops into four megaspores as a result of one reduction and one simple division.
Microspores and megaspores present first stage of gametophytic generation. Microspores form male gametophytes and megaspores female gametophyte.
Male gametophytes: Microspore is the unit of male gametocyte. Microspores are small and are surrounded by exine and intine spore coats. Exine being thick and worty white intine is thin. Each microspore has haploid nucleus and cytoplasm.
Microspores develop in micro-sporangium.
Microspore divides to small prothallial cells and larger antheridial cell which again divides into four altheridial cells which further divide by periclimal division to form four central androgonial cells, four jacket cells and one prothallial ell. Jacket cell further divides to form 8 jacket cells, 4 antdogonial cell one prothallial cell. Thus 13 celled male gametophytes are formed. Four anderogonial cells divide to form 256 androcytes or antherozoid mother cells the jacket cells. Each androcyte gets metamorphoid into an antherozoid which is spirally coiled and biciliate.
When antherozoids are mature, swelling of mucilage by absorption of water bursts the wall of microspore and biflagellate antherozoids get liberated free. They swim freely in search of female reproductive organs.
Megaspore is the unit of female gametocyte. Megaspores are bigger in size.
The wall of megaspore has thin intine and thick exine. Inside it is present a haploid nucleus and cytoplasm. Development starts when megaspores are present in mega-sporangium. Nucleus divides into large number of nuclei without and cell wall formation, after sometime it takes place and female gametophyte divides into upper region called Prothallus and lower storage tissue, between which lies diaphragm, female prothallus comes out and rhizoids arise from it. Few cells of female prothallus work as archegonial initials which divide to form four rows of cells, each row with two cell and venter. Venter has an egg and ventral canal cells and neck has neck canal cells.
Fertilization: water is necessary for fertilization. Antheridia pass down in neck of archegonium one antherozoid fuses with egg to form oospore. Fertilized oospore or zygote is the unit of sporophytic generation.