What is seed habit in Pteridophytes?
Heterospory and Seed Habit in Pteridophytes
• The occurrence of two kinds of spores in the same plant is called heterospory. Of the two kinds of spores, the smaller ones are called microspores and the larger ones are termed as megaspores and are produced within the microsporangia and megasporangia, respectively.
• The production of two kinds of spores in heterosporous plants is definitely related with a differentiation in sex since a microspore develops,a male gametophyte and a megaspore a female gametophyte. The heterospory, thus, is associated with the sexual differentiation of gametophyte. For More Biology Doubts visit main page of Physics Wallah.
• The differentiation of spores into microspores and megaspores and their dependence on the parent sporophyte for the nutrition are the certain features in the life cycle of Selaginella that have been considered as the essential prerequisites for the formation of seeds, characteristic of spermatophytes. It is generally agreed that the seed plants arose from the heterosporous vascular plants that instead of discharging the megaspore acquired the habit of retaining it within the megasporangium.
• In the seed bearing plants there are two kinds of spores: microspores and megaspores that grow to form male and female gametophytes, respectively; In these plants, the single megaspore is not shed from the megasporangium but is retained within it while still attached to the mother plant. It germinates inside the megasporangium (nucellus) producing the much reduced female gametophyte bearing the archegonia. Later the nucellus and the gametophyte are protected by a covering or integument and the whole structure is known as an ovule. The female gametophyte gets nutrition for its development from the parent plant and thus does not need to produce by its own effort. After fertilization the zygote within the ovule gives rise to an embryo, the rest of the gametophytic tissue gives rise to the nutritive tissue or endosperm and the integument thickens to form a seed coat. This entire structure, i.e., the integumented ovule is known as seed. It is detached from the parent plant and germinates to form a new plant.
• Thus we find that for the production of seeds, the following prerequisites are essential:
• Production of two types of spores (heterospores).
• Megasproangium (nucellus) does not open and the single megaspore within it germinates to from the female gametophyte.
• Nucellus becomes invested by a covering or integument which later forms a seed coat.
• Within the nucellus is formed a linear tetrad of four haploid megaspores as a result of the reduction division in the functional megaspore mother cell. Out of these four megaspores, the lowermost gives rise to the female gametophyte whereas the rest degenerate.
• The male gametes reach the egg by means of a tubular outgrowth of the male gametophyte known as the pollen tube.
• Fertilization and formation of embryo take place within the megasporangium.
• The embryo undergoes a resting period.
• Selaginella exhibits a remarkable approach to the seed habit the characteristic of the spermatophytes because of the following features:
• The heterospory occurs in almost all the species of Selaginella.
• In most species only one functional megaspore mother cell is produced which by reduction division.
• One megaspore is formed in each megasporangium and this single megaspore is not shed but germinates to form the female gametophyte.
• The fertilization and development of embryo in both the species take place while the megaspore is enclosed within the megasporangium.
• Therefore, it becomes evident that Selaginella is considerably advanced towards the seed habit in few species but its approach to the true seed is not completed due to the following features:
• The megasporangum lacks an integument or covering.
• The retention of the megaspore permanently within the megasporangium has not become evident.
• After the development of the embryo there is a lack of resting period.
• It is a good source of potash when burnt.
• Ferns are grown as ornamental plants because of their graceful foliage.
• An antihelmintic drug is obtained from rhizome and petiole of Dryopteris.
• The sporocarps of Marsilea are used as food by certain tribes.