PLS 002: Botany and physiology of cultivated plants · LibreTexts
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3.2.3: Seed Plants
Opening the book…
Fossil from the Devonian period reveal fernlike plants that were heterosporous; that is, produced two kinds of spores: microspores and megaspores. The megaspores were not released from the parent sporophyte. Fertilization took place within the tissue of the parent sporophyte thus freed from dependence on surface water. However, the necessity for the microspores to be carried from one plant to another in order to reach the megagametophyte robbed them of their value as agents of dispersal. This function was taken over by seeds - dormant, protected, embryo sporophytes.
Seeds represent one of the most important innovations in plant evolution: a protected, nutrient-supplied embryo with the ability to await appropriate conditions for germination. Seeds and pollen allowed plants to limit their reliance on water for completion of their life cycle. The first plants to evolve seeds were the gymnosperms, which grew wider and taller with secondary growth. Angiosperms then improved upon seed dispersal and pollination strategies with the evolution of fruits and flowers.
In gymnosperms, protective seeds filled with nutritive tissue (including the megagametophtye) replace spores as the dispersal mechanism. Antheridia are lost in the microgametophyte, which is reduced to four cells and is dispersed as a whole (pollen). The evolution of secondary growth allows for the lateral deposition of woody tissues. This latter development, along with xerophytic leaves, allows gymnosperms to tolerate a wide variety of new environmental stressors.
Angiosperms are plants that produce flowers and fruits. Within the ovule, double fertilization results in the formation of both the zygote and endosperm. New specialized cells are present in the vascular tissue. Meanwhile, the gametophytes are further reduced and archegonia are lost altogether. Nearly 90% of all plants belong to this group.
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