Agriculture Reference
In-Depth Information
Figure 2.6. Stylized drawing of the formation, development, and maturation of two ovules on the ovuliferous
scale of Douglas i r (after Allen and Owens, 1972).
female Gametophyte development
As in angiosperms, the female gametophyte development in gymnosperms requires two distinct processes:
megasporogenesis and megagametogenesis.
Megasporogenesis is similar in both angiosperms and gymnosperms. A cell within the nucellus of
the developing ovule enlarges. This cell, the megaspore mother cell or megasporocite, is 2n (diploid) and
undergoes meiosis, forming a tetrad of haploid (1n) cells, or megaspores. Three degenerate, leaving one
functional megaspore (Fig. 2.7).
Megagametogenesis is similar in angiosperms and gymnosperms in that a single haploid cell under-
goes development resulting in a female gametophyte consisting of the egg apparatus and nutritive tissues
(Fig. 2.7). However, the structures are distinctly different between the two classes.
The megaspore, surrounded by nucellar tissue within the ovule, undergoes several free-nuclear divi-
sions without cell wall formation. At the same time a nutritive tissue, called the
tapetum
, develops.
Following the free nuclear stage, cell walls form in the tissue of the female gametophyte, surrounded
by the megaspore wall and tapetum. As the female gametophyte develops, the tapetum degenerates.
Cells within the female gametophyte near the micropyle undergo changes, resulting in several (four
to six) archegonial initials. Subsequent nuclear division of the
archegonial
initials
results in a large central
cell and a smaller primary neck cell. The central cell enlarges and elongates toward the center of the female
gametophyte. Division of the central cell results in a large egg cell and a small ventral canal cell (Fig. 2.7).
The mature female gametophyte consists of many thin-walled haploid cells enclosing several archego-
nia. During pollination, an egg is fertilized and develops into an embryo surrounded by female gametophyte
tissue. The haploid
megagametophyte
serves as the nutritive tissue of gymnosperm seeds.
