Flora Unveiled

484 i Flora Unveiled

are, in fact, derived from two different types of spores, as in the heterosporous cryptogams,
flowering plants would be considered heterosporous, like Selaginella.

Hofmeister Unifies the Life Cycles of Cryptogams

and Seed Plants

There are two types of seed plants, the angiosperms, or flowering plants, and the gym-
nosperms, which include the conifers, cycads, Ginkgo, and the rarer group called the
Gnetophyta (Gnetum, Welwitschia, and Ephedra). The term angiosperm literally means
“vessel- seed,” based on the fact that the ovules (immature seeds) are enclosed by the carpel.
After fertilization, the ovule develops into the seed and the carpel forms the fruit. The term
gymnosperm literally means “naked- seed.” In Gymnosperms, the ovules are borne exposed
on the surfaces of highly modified leaves, such as the cone scales of conifers. The ovules of
both angiosperms and gymnosperms are fertilized by sperm cells delivered directly by pol-
len tubes. After fertilization, gymnosperm ovules develop into seeds, but since they are not
enclosed by a carpel (fruit tissue), they are borne “naked.” The shell surrounding a pine nut,
for example, is a seed coat rather than a fruit wall.^14
We have now arrived at the core of Hofmeister’s theory, which is based on the follow-
ing hypothesis: What if seed plants are like heterosporous ferns and fern allies, which
produce two types of asexual spores: male and female? Could the ovule and the anther
represent sporangia? If so, the ovule is actually a female sporangium or megasporan-
gium, while the anther is a male sporangium, or microsporangium. If these conjec-
tures are valid, the ovule should produce a megaspore, while the anther should produce
a microspore, demonstrating the unity of the life cycles of the cryptogams and seed
plants!
The supposition that the ovule is a megasporangium and the anther is a microsporan-
gium is counterintuitive. Based on ferns and other cryptogams, we think of spores as asex-
ual reproductive structures that are released from the plant and germinate on the ground.
Hofmeister proposed the following radical hypothesis: what if the spores of seed plants are
not released from the plant? What if they divide, develop, and mature into multicellular
male and female gametophytes internally, within the sporophytic tissues of the anthers and
ovaries? This would mean that the embryo sac and pollen grains are actually highly reduced
male and female gametophytes, distinct from the much larger sporophytic parent that pro-
duced them, just as the tiny gametophytes of mosses and ferns (the prothalli) are separate
plants from their sporophytic “parent.”
This revolutionary theory is shown schematically in Figures 18.4 and 18.5, with the struc-
tures colored green representing the sporophyte and the structures in red representing the
gametophyte generations (see color inserts). The left side of Figure 18.4 shows a sporophytic
anther in green with a cutaway view of the interior showing the gametophyte generations
(pollen grains) in red. In the two cross- sections on the right, the upper cross- section is an
immature anther. At this early stage, the microsporangium is filled with “sporogenous cells”
that will later give rise to spores. The lower cross- section shows a mature stage containing
pollen grains. The pollen grains are colored red because they represent male gametophytes,
while the surrounding tissues are green because they are sporophytic tissue.