The basic characteristics required in plants for development
of seed structures may be summarized as follows:
Phanerogams possess all these characteristics and hence can produce seeds.
Click here to enlarge
Figure 15.26 Stages in the development of seed structure
(A) Selaginella. Female gametophyte with
the embryo is within megasporangium
(B) Longitudinal Section of the ovule of a fossil seed fern
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
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).
(B) Seeds in relation to terrestrial habitat
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.
SUMMARY- SEED HABIT
Evolution of the seed is one of the most advantageous
adaptations in Phanerogams to combat terrestrial habitat. It
gives full protection and parental care to the embryo. Allowing
for a dormancy period, germination of the seed occurs only under
favorable conditions, enhancing survival of the new plant. Heterospory
was the first step in the evolution of seeds. Seed develops
from the ovule only after fertilization.