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The
Diversity of The World of Life
Green
Plants (Viridaeplantae)
Cycads
(Cycadophyta, or
Cycadales)
Representatives
Cycads,
including Sago "Palm"
Biology
ENVIRONMENTS
Cycads live on land, in certain Tropical and Subtropical areas.
Cycads were an abundant source of food during the Age of Dinosaurs.
OVERALL STRUCTURE
Cell walls, composed primarily of cellulose, give shape to individual cells.
Cycads are fairly large-sized plants with woody roots,
palm-like woody stems (mostly underground in some species), fern-like leaves, and
pine-like cones.
ENERGY CAPTURE
Light-energy is captured, for photosynthesis, by
chloroplasts, within the cells in the leaves.
EXCHANGE
OF MATERIALS WITH THE ENVIRONMENT
Water vapor and gases flow especially through
"stomata" pores (each regulated by a pair of "guard
cells") in leaves. A
waxy "cuticle" helps prevent water loss from the leaves, and woody
tissues help prevent water loss from the stem.
Water with dissolved substances is absorbed especially by the
fine "root hairs", at the tips of young, "primary" roots.
INTERNAL TRANSPORT
Presumably
like that in conifers.
DEVELOPMENTAL CONTROL
The growth and development of cycads is under genetic and
undoubtedly hormonal control.
ASEXUAL REPRODUCTION
Cycads can reproduce asexually, via vegetative body parts.
SEXUAL REPRODUCTION
In
cycads, as in other plants, there is an "alternation of
generations" in the life cycle, between "diploid" forms
(with both sets of chromosomes) and "haploid" forms (with just
one set of chromosomes).
As with other higher plants, the reduced, haploid
"gametophytes" of cycads are
dependent on the dominant, diploid "sporophytes"
(the typical plant bodies).
The
life cycle of cycads is similar to that of conifers.
Within the "ovulate" (female) cones of cycads are
"ovules", each
consisting of protective "integuments" covering a "megasporangium" (the
"nucellus" tissue), within which forms the haploid female "gametophyte".
The female gametophyte bears much reduced "archegonia", each
containing an egg (the female "gamete"). "Microsporangiate"
(male) cones release haploid "microspores", carried by
the wind to female cones. After
being drawn into the "micropyle" (entryway) of the ovule, a
microspore develops into the mature male gametophyte -- a pollen grain with
pollen tube, growing into the nucellus and releasing its flagellated
sperms. The sperms swim through fluid in the nucellus and into an archegonium (Significantly,
environmental water is not required for sexual fertilization, as it is for
lower plants -- higher plants, like higher animals, are better adapted to
life on dry land). One of the
sperms fertilizes the single egg in the archegonium, and the diploid
"zygote" (fertilized egg) develops into the embryo sporophyte.
The integuments (as a "seed coat") continue to cover the female gametophyte,
which will be consumed by the embryo within, after a period of dormancy:
A true seed is thus formed.
Green
Plants (Viridaeplantae)
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