homosporous
. After germinating from a spore, the gametophyte produces both male
and female
gametangia
, usually on the same individual. In contrast,
heterosporous
plants produce
two morphologically different types of spores. The male spores are known as microspores due to their
small size; the larger megaspores will develop into the female gametophyte. Heterospory can be found
in a few seedless vascular plants as well as all seed plants.
Mitosis produces a multicellular gametophyte when the haploid spore germinates. The
gametophyte feeds the zygote formed by gamete fusion and the resulting young sporophyte or
vegetative form, and the cycle is restarted (Figure 83 and Figure 84).
58
credit: modification of work by Peter Coxhead
133
Figure 83.
A fern's life cycle exhibits generational alternation with a dominant sporophyte stage
59
.
The spores of seedless plants and pollen of seed plants are surrounded by thick cell walls that
contain
sporopollenin
, a tough polymer. This substance, which gives most pollen its yellow color, is
made up of long chains of organic molecules related to fatty acids and carotenoids. Sporopollenin is
chemically and biologically resistant to a high degree. Its toughness explains the existence of well-
preserved fossils of pollen. Sporopollenin was once thought to be an innovation of land plants;
however, the green algae
Coleochaetes
is now known to form spores that contain sporopollenin. The
protection of the embryo is critical for land plants. Desiccation and other environmental hazards must
be avoided for the vulnerable embryo. The female gametophyte provides nutrition in both seedless
and seed plants, and the embryo in seed plants is also protected as it develops into the next generation
of sporophytes
60
.
59
credit "fern": modification of work by Cory Zanker; credit "gametophyte": modification of work by
"Vlmastra"/Wikimedi
60
Fowler, S.,Roush, R. & Wise, J. (2017)
Concepts in Biology,
Chapter 14, -Pp. 326-328 OpenStax,
https://openstax.org/details/books/concepts-biology
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