Heterosporous refers to the plants producing different types of spores that are different in structure, size and function. They produce two kinds of spores, smaller microspores and bigger megaspores.
Most pteridophytes are homosporous, i.e. they produce all spores of a similar kind. Some pteridophytes’ genera like Selaginella and Salvinia are heterosporous.
Heterospory is important with respect to the evolutionary perspective. Heterospory is found in some pteridophytes and all seed plants, i.e. gymnosperms and angiosperms.
Salient features of Heterospory in Pteridophytes
Heterospory first evolved in some pteridophytes. In heterosporous plants, two different kinds of spores are produced having different sexes.
- The smaller spores are known as microspores and the larger ones are known as megaspores.
- Microspores are present in microsporangia, whereas megaspores are present in megasporangia.
- Microspores are smaller in size but more in number, whereas megaspores are larger in size but less in number.
- In Selaginella, all the microsporocytes or microspore mother cells undergo meiosis to produce a large number of microspores, whereas only one megasporocyte is functional and produces four large megaspores after meiosis.
- Heterospory becomes evident before meiosis as in Selaginella or after meiosis as in Marsilea.
- The origin of heterospory is related to nutritional factors.
- Functional megaspores accumulate the nutrients and the rest disintegrate.
- Pteridophytes show alternation of generations between diploid sporophytes and haploid gametophytes. The sporophyte is the dominant phase in the life cycle of pteridophytes.
- In the homosporous species, sex differentiation takes place at the gametophyte stage but in the heterosporous species, sexes are separated at the sporophytic stage itself.
- Microspore gives rise to a male gametophyte and megaspore gives rise to a female gametophyte. The germination of microspores and megaspores initiates within the sporangium.
- Heterospory is a significant evolutionary step leading to seed habits.
- Megaspores germinate inside the sporangium to form a female gametophyte.
- Female gametophytes are retained inside the sporophytes. Fertilisation of the egg, development of the zygote into a young embryo takes place within the female gametophyte.
- The female gametophyte derives its nutrients from the sporophyte.
Examples of Heterosporous Pteridophytes
Heterospory is seen in 9 genera of pteridophytes. They are:
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