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No evolutionary event in the colonization of the land by vascular plants is of greater significance than the appearance of the seed habit. What apparently began as an adaptation enhancing sexual reproduction in the absence of external free water has assumed a far broader importance in terms of survival and dispersal. The essential features of the seed habit are identified in relation to heterospory, megaspore retention, and endosporic gametophyte development and approaches to the seed habit by nonseed plants are reviewed. Seeds first appear in the fossil record of the Late Devonian Period and, in diverse forms and sizes, constitute a significant component of Carboniferous fossil floras. The rise to dominance of plants of gymnospermous affinity in the early Mesozoic and the appearance and rapid adaptive radiation of the angiosperms are discussed in relation to the significance of the seed habit. The structure and development of modern gymnosperm and angiosperm seeds are compared, and the contrasting patterns of embryogenesis in these two types of seeds are interpreted in terms of the relationship between the embryo and its nutritive support system.
Taylor A. Steeves (Thu,) studied this question.