This event is a precursor to the seed habit considered an important step in evolution.
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Heterospory is a phenomenon in which two kinds of spores are borne by the same plant. These spores differ in size. The smaller one is known as microspore and the larger one is known as megaspore. The microspore germinates to form the male gametophyte and the megaspore germinates to form the female gametophyte. Both these gametophytes are retained on the parent sporophyte in the respective sporangia. The male gametophyte releases the male gametes and these reach the female gametophyte to fuse with the egg. The development of the zygote takes place inside the female gametophyte.
This retention and germination of the megaspore within the megasporangium ensures proper development of the zygote. The zygote develops into the embryo that is future sporophyte. The evolution of the seed habit (an evolutionary mechanism of development of seeds) is related to the retention of the megaspore.
Heterospory is thus considered an important step in evolution as it is a precursor to the seed habit. Heterospory evolved first in pteridophytes such as Selaginella and Salvinia. is a phenomenon in which two kinds of spores are borne by the same plant.