The structure are as given
Heterospory is production of spores of two different sizes and sexes by the sporophytes of land plants. Two types of spores are produced by heterosporic plants.
Small spores are microspores which germinate into male gametophyte and large spores are macrospores which develop into female gametophyte.
In evolution of plants pteridophytes are intermediate between bryophytes and gymnosperms. All bryophytes are homosporous and all gymnosperms are heterosporous. This condition is advanced as sexual dimorphism result in cross fertilisation.
Primitive or earlier pteridophytes are homosporous later pteriodophytes are heterosporous e.g., Dryopteris, Pteris-homosporous Selaginella, Salvinia-heterosporous.
Seed habit 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, which have been considered as the essential pre-requite for formation of seed, the characteristic of spermatophyte.
The evolution of heterospory and seed habits in Selaginella is evident by the following characters
(i) Reduction to a single functional megaspore per sporangium.
(ii) Retention and germination of megaspore within megasporangium
(iii) Development of protective layer and nutritive tissue called tapetum is present.
(iv) Development of embryosac with in the sporangium.
(v) Modification of distal end of mega sporangium to capture pollen grain.
(vi) Pollination and siphonogamy.
(vii) Temporary suspension of growth of embryo (dormancy period).
Cycas as the relic of past Cycas is an evergreen plant which looks like a palm. It has unbranched stem and large compound leaf. It exhibit phylogenetic relationship with pteridophyte. Its evolutionary characters are
(i) Slow growth.
(ii) Shedding of seed when the embryo is still immature.
(iii) Little secondary growth and manoxylic wood.
(iv) Leaf like megasporophyllus.
(v) Flagellate sperms even when pollen tube is present.
(vi) Persistent leaf bases.
(vii) Circinate ptysix.
(viii) Arrangement of microsporangia is well defined archegonia.