Flora Unveiled

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Flora’s Secret Gardens j 485

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Figure 18.5 shows the pistil with ovules (megasporangia) inside the ovary, all of which
are sporophytic tissue and therefore colored green. Four stages of ovule development are
shown. The immature ovule is entirely green, including the megaspore mother cell, because
it has not yet divided to produce megaspores. In the second stage, four megaspores have
been produced, only one of which will survive. Since these represent the first stage of the
gametophyte generation, they are colored reddish brown. In the third stage, the single func-
tional megaspore has undergone two consecutive nuclear divisions to produce the four-
nucleate stage of the embryo sac, or immature female gametophyte. During this time, the
ovule gradually reorients itself 180 degrees. In the fourth stage, the nuclei of the embryo sac
have undergone a final nuclear division to produce eight nuclei altogether. However, two of
them subsequently fuse to form the single large nucleus of the central cell in the middle of
the embryo sac. Cell membranes then form around each of the seven nuclei to produce the
mature female gametophyte or embryo sac.
Hofmeister did not simply propose a hypothesis; he provided abundant evidence to sup-
port it. He showed that in the conifers (gymnosperms), a megaspore cell within the ovule
divides to form a multicellular female gametophyte, which includes archegonia containing
egg cells.^15

Figure 18.4 Anther, or microsporangium. The sporophyte generation is colored green and
the gametophyte generation is colored red. A three- dimensional view of the mature anther
is shown on the left, and two cross- sections representing the immature anther (top) and the
mature anther (bottom) are shown on the right. The upper section is entirely green because
the sporogenous cells in the four lobes have not yet undergone meiosis (discussed in the next
section) to produce the haploid microspores, which represent the first stage of the gametophyte
generation. The lower section shows a mature anther containing pollen grains, representing the
male gametophyte. See color insert.
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