The Questions of Developmental Biology

eproductive (flower-producing) development (see reviews by McDaniel et al. 1992 and Levy and
Dean 1998). Once the meristem becomes floral, it initiates the development of floral parts
sequentially in whorls of organs modified from leaves (Figure 20.5). The first and second whorls
become sepals and petals, respectively; these organs are sterile. The pollen-producing stamens
are initiated in the third whorl of the flower. The carpel in the fourth whorl contains the female
gametophyte. The stamens contain four groups of cells, called the microsporangia (pollen sacs),
within an anther. The microsporangia undergo meiosis to produce microspores. Unlike most
ferns, angiosperms are heterosporous, so the prefix micro is used to identify the spores that
mitotically yield the male gametophytes pollen grains. The inner wall of the pollen sac, the
tapetum, provides nourishment for the developing pollen.
Pollen
The pollen grain is an extremely simple multicellular structure. The outer wall of the pollen grain,
the exine, is composed of resistant material provided by both the tapetum (sporophyte generation)
and the microspore (gametophyte generation). The inner wall, the intine, is produced by the
microspore. A mature pollen grain consists of two cells, one within the other (Figure 20.6). The
tube cell contains a generative cell within it. The generative cell divides to produce two sperm.
The tube cell nucleus guides pollen germination and the growth of the pollen tube after the pollen
lands on the stigma of a female gametophyte. One of the two sperm will fuse with the egg cell to
produce the next sporophyte generation. The second sperm will participate in the formation of the
endosperm, a structure that provides nourishment for the embryo.
The ovary
The fourth whorl of organs within the flower forms the carpel, which gives rise to the female
gametophyte (Figure 20.7). The carpel consists of the stigma (where the pollen lands), the style,
and the ovary. Following fertilization, the ovary wall will develop into the fruit. This unique
angiosperm structure provides further protection for the developing embryo and also enhances
seed dispersal by frugivores (fruit-eating animals). Within the ovary are one or more ovules
attached by a placenta to the ovary wall. Fully developed ovules are called seeds. The ovule has
one or two outer layers of cells called the integuments. These enclose the megasporangium,
which contains sporophyte cells that undergo meiosis to produce megaspores (see Figure 20.4).
There is a small opening in the integuments, called the micropyle, through which the pollen tube
will grow. The integuments an innovation first appearing in the gymnosperms develop into
the seed coat, which protects the embryo by providing a waterproof physical barrier. When the
mature embryo disperses from the parent plant, diploid sporophyte tissue accompanies the
embryo in the form of the seed coat and the fruit.
Within the ovule, meiosis and unequal cytokinesis yield four megaspores. The largest of these
megaspores undergoes three mitotic divisions to produce a seven-celled embryo sac with eight
nuclei (Figure 20.8). One of these cells is the egg. The two synergid cells surrounding the egg
may be evolutionary remnants of the archegonium (the female sex organ seen in mosses and
ferns). The central cell contains two or more polar nuclei, which will fuse with the second sperm
nucleus and develop into the polyploid endosperm. Three antipodal cells form at the opposite
end of the embryo sac from the synergids and degenerate before or during embryonic
development. There is no known function for the antipodals. Genetic analyses of female
gametophyte development in maize and Arabidopsis* are providing insight into the regulation of
the specific steps in this process (Drews et al. 1998).