486 i Flora Unveiled
In angiosperms, he demonstrated that the inner tissues of the ovule give rise to a single
functional megaspore (see Figure 18.5), which develops into a highly reduced female game-
tophyte generation, or embryo sac. At maturity, the female gametophyte of angiosperms is
called the embryo sac, consisting of an egg cell plus several other types of cells. As we shall
see in the next section, this tiny gametophyte is genetically distinct from the ovule that
produced it, so it truly represents a separate “generation.”
Compared to the prothalli of heterosporous ferns and fern allies, this female gameto-
phyte is so reduced in size that it doesn’t even produce an archegonium. In the course of
evolution, the female gametophyte generation of angiosperms has been reduced to its core
function: egg production.
The microspores of flowering plants are formed in the anthers, which are the microspo-
rangia of angiosperms. The highly reduced male gametophyte, the pollen grain, consists
only of a vegetative, or tube cell that regulates the growth of the pollen tube, plus two sperm
cells. As in the case of the female gametophyte, the male gametophyte of flowering plants
has been reduced to its core functions, sperm production and transport. Unlike the embryo
sac, however, the pollen grain escapes the confines of its microsporangium and is carried by
wind or insects to the stigma (usually from another flower), where it promptly sends down
a pollen tube that grows down through the style. Upon penetrating the embryo sac, the
tip bursts, releasing the two sperm cells, one of which fuses with the egg cell. An overview
of the angiosperm life cycle illustrating alternation of generations is shown in Figure 18.6.
Hofmeister correctly deduced the basic outline of alternation of generations, despite the
fact that many of the details had not yet been elucidated. For example, Hofmeister had
not actually observed the fusion of egg and sperm. In his studies with angiosperms, he
observed the pollen tube tip reaching the embryo sac in the ovule, but he never actually saw
F igu r e 18.5 Ovule, or megasporangium. The diagram of a pistil on the left shows a longitudinal
section through the ovary, with the ovules attached to a central structure called the placenta, a term
borrowed from mammalian biology. The developmental sequence begins with the immature ovule
colored entirely green because it is prior to the formation of the megaspore. Meiosis (discussed
later) gives rise to the four haploid megaspores (colored reddish brown), only one of which survives.
As the ovule undergoes further development, the functional megaspore undergoes three nuclear
divisions to produce the mature female gametophyte, or embryo sac. The embryo sac contains the
egg. See color insert.