The Questions of Developmental Biology

oth the embryo and the mature sporophyte are nourished by the gametophyte. Meiosis within the
capsule of the sporophyte yields haploid spores that are released and eventually germinate to form
a male or female gametophyte.
Ferns follow a pattern of development similar to that of mosses, although most (but not all) ferns
are homosporous. That is, the sporophyte produces only one type of spore within a structure
called the sporangium (Figure 20.3). One gametophyte can produce both male and female sex
organs. The greatest contrast between the mosses and the ferns is that both the gametophyte and
the sporophyte of the fern photosynthesize and are thus autotrophic; the shift to a dominant
sporophyte generation is taking place.
At first glance, angiosperms may appear to have a diplontic life cycle because the gametophyte
generation has been reduced to just a few cells (Figure 20.4). However, mitotic division still
follows meiosis in the sporophyte, resulting in a multicellular gametophyte, which produces eggs
or sperm. All of this takes place in the the organ that characterizes the angiosperms: the flower.
Male and female gametophytes have distinct morphologies (i.e., angiosperms are heterosporous),
but the gametes they produce no longer rely on water for fertilization. Rather, wind or members
of the animal kingdom deliver the male gametophyte pollen to the female gametophyte.
Another evolutionary innovation is the production of a seed coat, which adds an extra layer of
protection around the embryo. The seed coat is also found in the gymnosperms. A further
protective layer, the fruit, is unique to the angiosperms and aids in the dispersal of the enclosed
embryos by wind or animals.
The remainder of this chapter provides a detailed exploration of angiosperm development from
fertilization to senescence. Keep in mind that the basic haplodiplontic life cycle seen in the
mosses and ferns is also found in the angiosperms, continuing the trend toward increased
nourishment and protection of the embryo.
*Have you ever wondered why there are no moss trees? Aside from the fact that the
gametophytes of mosses (and other plants) do not have the necessary structural support and
transport systems to attain tree height, it would be very difficult for a sperm to swim up a
tree!
It is possible to have tree ferns, for two reasons. First, the gametophyte develops on the
ground, where water can facilitate fertilization. Secondly, unlike mosses, the fern sporophyte has
vascular tissue, which provides the support and transport system necessary to achieve substantial
height.
Gamete Production in Angiosperms
Like those of mosses and ferns, angiosperm gametes are produced by the gametophyte
generation. Angiosperm gametophytes are associated with flowers. The gametes they produce
join to form the sporophyte. The study of embryonic development in plants is therefore the study
of early sporophyte development. In angiosperms, the sporophyte is what is commonly seen as
the plant body. The shoot meristem of the sporophyte produces a series of vegetative structures.
At a certain point in development, internal and external signals trigger a shift from vegetative to