132
Figure 82.
Generational switch between haploid (1n) gametophyte and diploid (2n) sporophyte
58
.
A life cycle with a dominant haploid stage is referred to as haplontic.
Diplontic
refers to a life
cycle in which the diploid stage is dominant and the haploid chromosome number appears only briefly
during sexual reproduction.
Humans are diplontic, for example. Most plants exhibit alternation of
generations, which is described as
haplodiplontic
: the haploid multicellular
form known as a
gametophyte is followed in the development sequence by a multicellular diploid organism, the
sporophyte
. The
gametophyte
gives rise to the gametes, or reproductive cells, by mitosis. It can be
the most visible stage of the plant's life cycle, as in mosses, or it can occur in a microscopic structure,
such as a pollen grain in higher plants (the collective term for the vascular plants). In lower plants,
the sporophyte stage is barely discernible (the collective term
for the plant groups of mosses,
liverworts, and hornworts). Towering trees are the diplontic stage of plant lifecycles such as sequoias
and pines.
Sporangia in the Seedless Plants
The seedless plant's sporophyte is diploid and the result of
syngamy
, or the fusion of two
gametes (Figure 82). The sporophyte has
sporangia
(plural, sporangium), which are organs that first
appeared in land plants. Sporangia literally means "spore
in a vessel," as it is a reproductive sac
containing spores. Inside
the multicellular sporangia, diploid sporocytes,
or mother cells, meiosis
produces haploid spores, reducing the 2n chromosome number to 1n. The sporangia then release the
spores, which disperse in the environment. Land plants produce two types of spores, resulting in the
separation of sexes at different points in the life cycle. Seedless nonvascular plants (also known as
"seedless nonvascular plants with a dominant gametophyte phase") produce only one type of spore,
and are called
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