Flora Unveiled

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480 i Flora Unveiled


By 1847, at the age of twenty- three, Hofmeister had begun the series of studies that
was to become the foundation of his revolutionary theory of the alternation of gen-
erations. This research was published in book form by his father’s firm in 1851, under the
title: Comparative Investigations on the Germination, Development and Fruit Formation of
the Higher Cryptogams and Seed- formation in Conifers.
Based mainly on freehand sections of fresh material, Hofmeister was the first to recog-
nize that the life cycles of all the cryptogams, including the mosses, liverworts, hornworts,
ferns, and related species, alternate between two distinct multicellular generations: a gamete-
producing generation that begins with a spore, and a spore- producing generation that begins
with a fertilized egg.^10 The gamete- producing generation, or gametophyte (“gamete- plant”),
contains the sexual reproductive structures (antheridia and archegonia) that produce the
sperm and egg cells, respectively. The spore- producing generation, or sporophyte (“spore-
plant”), contains the asexual reproductive structures (sporangia) that produce the spores.
Now let’s see how Hofmeister’s general scheme for alternation of generations plays out
in the cryptogams, starting with the mosses. As shown in Figure 18.1, asexual spores are
released from the moss capsule, part of the sporophyte stage, which grows out of the top
of the leafy gametophyte stage. The spores then germinate to form a filamentous proto-
nema, which develops buds along its length. These buds grow to form the mature leafy
gametophyte stage.
The sexual structures of mosses, the antheridia and archegonia, are located at the tips
of the leafy gametophytes. The antheridia produce motile sperm cells. These motile sperm
cells are picked up by splashing raindrops, and droplets containing sperm cells fall onto the
archegonium. After being deposited near the mouth of the archegonium, they swim down
the narrow canal and fertilize the egg cell situated at the base. The fertilized egg is termed
the zygote. Finally, the zygote begins dividing and develops into the new mature sporophyte,
and the cycle begins over again.
The sporophyte of mosses is called a “dependent” sporophyte because it remains attached
to the leafy gametophyte its entire life. As shown in the diagram, spores are produced by a
special type of cell division called meiosis, but since meiosis hadn’t yet been discovered when
Hofmeister published his work on alternation of generations, we’ll defer our discussion of
this topic until later in the chapter.
A similar, but somewhat different pattern is observed in ferns, which are vascular
plants— that is, plants with specialized conducting tissues for water, minerals, and sug-
ars. The vascular systems of fern sporophytes allows them to grow to a considerable size
and become independent plants, in contrast to the dependent sporophytes of mosses. As
shown in Figure 18.2, asexual spores are released from sporangia located on the under-
sides of fern leaves. Leaves that produce spores are called sporophylls (spore- leaves). The
released spores germinate on the ground and divide to form the small flat prothallus, or
mature gametophyte generation. Two types of gamete- forming structures (gametangia,
or “gamete- vessels”) are formed on the underside of the sexual stage, the prothallus: male
antheridia and female archegonia. The antheridia produce motile sperm cells, and the
archegonia contain single egg cells within their swollen bases. Since the antheridia and
archegonia are located on the underside of the prothallus, they are in direct contact with
a thin film of water at the soil surface. Upon being released, the motile sperm cells swim
to the archegonia and fertilize the egg cells. Each fertilized egg becomes a zygote, and
the zygote divides and develops into the young sporophyte, the asexual stage, within the

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