Agriculture Reference
In-Depth Information
AnGIoSPErM SEEd
Angiosperms are the most numerous and widely distributed of all plant kinds. They are characterized by
seeds that are covered or enclosed in an ovary. Angiosperms number about 220,000 species and comprise
the most numerous group of economic plants (crops and weeds). Their seeds are usually, though not always,
the product of double fertilization within the ovary, which will be described later in this chapter.
The stylized diagram of an angiosperm l ower in Figure 2.1 shows the sexual parts of the l ower con-
sisting of the
stamens
, or male pollen-bearing structures, and the
pistil
, or female ovule-bearing structure.
It is within the
ovary
, the swollen part of the pistil at the base of the style, that seed formation and develop-
ment takes place.
It should be recognized that the different processes of seed development occur together and in harmony
with one another. An ovary is not formed i rst and then “i lled” with the ovules which, when fertilized, grow
into seeds. However, for the sake of studying seed development, these processes will be discussed as one
developmental step at a time.
Seed development can be divided into the following i ve processes: (1) ovule development, (2) embryo
sac and pollen grain development, (3) pollination and fertilization, (4) embryo development, and (5) endo-
sperm development.
Figure 2.1. Diagrammatic sectional view of a l ower to show various parts; note the germination of pollen
grains, path of pollen tube, and its entry into embryo sac: (an) anther; (ant) antipodal cells; (eg) egg; (es)
embryo sac; (i ) i lament; (fn) funiculus; (ii) inner integument; (nu) nucellus; (oi) outer integument; (ov)
ovule; (ovr) ovary; (pd) pedicel; (pe) petal; (pg) pollen grain; (pl) placenta; (pn) polar nuclei; (pr) pericarp;
(pt) pollen tube; (re) receptacle; (se) sepal; (sg) stigma; (st) stamen; (sy) synergid; (sty) style. The pistil is
comprised of ovary, style and stigma (drawing by Sabry Elias).
