Q.Fusion of two motile gametes which are dissimilar in size is termed as

Most spore-producing plants make just one kind of spore, and every spore looks and behaves the same. Heterospory is the departure from that plan: it means producing two different kinds of spores instead of one. The word itself carries the idea — hetero for different, spory for spores. Picture a factory that used to turn out one standard part now making two distinct sizes, each destined for a different job; that is the shift heterospory describes.

The two kinds of spores are distinguished mainly by size and by what they grow into:

• Megaspores are the larger spores. They germinate into the female gametophytes.
• Microspores are the smaller spores. They germinate into the male gametophytes.

Plants that make only one type of spore are called homosporous, and this is the usual condition among pteridophytes. But a few genera break the pattern. Among the pteridophytes, Selaginella and Salvinia are heterosporous, and all the seed plants — the gymnosperms and angiosperms — are heterosporous too.

Why does this small change matter so much? In the heterosporous pteridophytes, the female gametophytes are not shed but are held on the parent sporophyte for varying lengths of time, and the zygote develops into a young embryo while still enclosed within that female gametophyte on the parent. This retention and development on the parent plant is regarded as a forerunner of the seed habit — an important step toward the seeds that later define the gymnosperms and angiosperms.