Agriculture Reference
In-Depth Information
ovule development in Angiosperms
The i rst signs of ovule development occur as small protrusions from the ovary
placenta
. With contin-
ued growth, these
ovule primordia
develop specialized tissue known as the
nucellus
which at l owering
time provides a site for sexual fusion and nourishment for the developing embryo. Soon, small secondary
outgrowths begin to develop on either side of the nucellus. With further growth and differentiation, these
outgrowths (the
integuments
) continue to enlarge and eventually completely surround the fully functional
nucellus. In some seeds, only one integument is present.
The fully mature ovule consists of one or two integuments that enclose the
nucellus
. The point at
which the integuments meet forms a small opening called the
micropyle,
which provides an entry to the
nucellus between the integuments. The ovule is attached to the placenta by a stalk called the
funiculus
. In
many species, the funiculus separates from the seed at maturity, leaving a scar called the
hilum
.
Within the nucellus, one cell, the
archesporial cell
(2n), develops characteristics that distinguish it
from adjacent cells. It soon divides into the
megaspore mother cell
(2n) and the
parietal cell
(2n). The
later usually degrades. The megaspore mother cell divides meiotically (reduction division) to form four
1n
megaspores
, one of which normally remains functional (Fig. 2.2). There are three types of
megasporo-
genesis
(embryo sac formation):
monosporic
(polygonum type), in which the embryo sac develops from a
Figure 2.2. Stylized diagram of megasporogenesis, within a developing ovule: (A) the archesporial cell in the
enlarging nucellus, (B) the division of the archesporial cell into the parietal cell and megaspore mother cell;
(C) the i rst meiotic division of the megaspore mother cell resulting in two daughter cells; (D) the result of
the second meiotic division resulting in four megaspores; (E) an enlarged view of the ovule showing three
megaspores that degenerate and the remaining functional megaspore that will give rise to the embryo sac
(drawing by Sabry Elias).
