(A) Selaginella. Female gametophyte with the embryo is within megasporangium

(B) Longitudinal Section of the ovule of a fossil seed fern e.g. Laginostoma

Pteridophytes are non-seeded plants. Most of them are homosporous. However, a few pteridophytes are heterosporous e.g. Selaginella, Isoetes, Marsilea, Azolla, Salvinia,. Some of these possess some of the characteristics needed for seed formation. For example, a species of Selaginella possesses the first four of the above mentioned requirements. However, the last four requirements are missing and hence Selaginella fails to produce seeds.

Gymnosperms and angiosperms are the seed bearing plants (Phanerogams). In these plants, megasporangium (nucellus) is surrounded by the protective integument and is called an ovule. In gymnosperms, it is directly borne by the megasporophyll (open carpel) and is called a naked ovule. It develops into a naked seed after fertilization.

The carpel in angiosperms is differentiated into ovary, style and stigma. Ovules are encased in an ovary cavity (closed carpel). After double fertilization, the ovule forms a seed and the ovary forms the fruit (see chapter units 15.4 and 15.5). Hence, seeds are very well protected and the embryo in the seed gets proper parental care, protection and nourishment.

During seed development, the zygote forms an embryo, and integuments form a seed coat. In gymnosperms, the female gametophyte acts as endosperm. Hence, it is a haploid and pre-fertilization tissue. In angiosperms, endosperm is a triploid and post-fertilization tissue.

A mature seed of gymnosperms (Fig. 15.14. B) is endospermous and the embryo may have two cotyledons (e.g. Cycas) or many cotyledons (e.g. Pinus).

In angiosperms, mature seed (figs 15.19 B and 15.22C) may be endospermic (e.g. sorghum, corn) or non-endospermic (e.g. pea, bean). The embryo may be monocotyledonous (sorghum, corn) or dicotyledonous (sunflower, pea, Hibiscus).

The development of seed structure was one of the biggest advances in the process of plant evolution. The terrestrial habitat provides widely fluctuating environmental conditions, which may not be always suitable for the growth and development of an embryo into a new plant. The seed provides necessary protection and care to the embryo. In seeds, growth of an embryo is takes place only when external conditions are favorable. This increases the chances of survival of the new individual.

Seeds of different plants are adapted for dispersal by wind, water, animals, etc. Hence they act as highly efficient vehicles of dispersal and propagation. The resistant seed coat protects the inner embryo from extremes of external conditions e.g. fluctuations in temperature, water, oxygen, light, etc. Moreover, the internal structures of a seed, especially the embryo are physiologically adapted to sustain prolonged adverse conditions of water, temperature, etc.