HETEROSPORY AND SEED HABIT HETEROSPORY • The phenomenon where two types of spores differing in size, structure and function are formed on the same plant is known as heterospory. • The smaller spores are called microspores and the larger spores are known as megaspores. • Heterospory has not evolved in living forms but was also present in fossil plants, and • It originated due to disintegration of some spores in a sporangium IMPORTANCE OF HETEROSPORY • Heterospory expresses sex determining capability of the plant e.g. a microspore always gives rise to male gametophyte and a megaspore to female gametophyte. • In heterosporous forms development of gametophyte is endosporic and the nutrition for the developing gametophyte is derived from the sporophyte. So the development of gametophyte is not affected by ecological factors as in case of independently growing gametophytes. • Megaspore is retained by the parent even after fertilization. This ensures nutrition for the developing embryo, • SEED HABIT • The requirements for the formation seed are as follows- • Formation of two types of spores microspores and megaspores (heterospory) • Reduction in the number of functional megaspores to one per megasporangium. • Retention of megaspore in the megasporangium until embryo development. • Elaboration of the apical part of megasporangium to receive microspores or pollen grains. • In Selaginella, the most common genus of the heterosporous pteridophytes, provides the best examples. Most of the species of Selaginella are heterosporous and they have only one functional megaspore mother cell which gives rise to 4 megaspores after meiosis. Only a single functional megaspore in a sporangium is present in Selaginella rupestris, S. monospora and S. erythropus. The development of female gametophyte, fertilization and embryo development takes places within the megasporangium. Thus, evolution of seed habit took place in such species of Selaginella. • However the seeds developed in these species cannot be called true seeds because (i) the megasporangium is not covered with integuments and (ii) there is not resting stage after embryo development ; the development of embryo is accompanied with the development of shoot and rhizophore. •.