Spermatophyta: seed plants
Seed plants consist of approximately 1,000
species of non-angiosperms (gym-nosperms) and about 250,000 species of
angiosperms. They have a sporic life-cycle with sporophyte predominance, and
seeds. The gametophyte is reduced to cells inside the ovule or pollen grain.
Males have a minimum number of cells being three and females being four. The
anteridia are absent and in flowering plants (Angiospermae) and Gnetopsida the
archegonia are also reduced. The sporophyte will always start as an embryo
located inside the nutrition tissue, endosperm1 which is the female
gametophyte or in endosperm2. Spermatophyta have axillary buds (buds
in leaf axils). Like ferns, they are megaphyllous and homoiohydric, and have a
secondary thickening. Higher groups of seed plants lost flagellate spermatozoa
and developed pollen tubes. The classes of Spermatophyta are Ginkgoopsida,
Cycadopsida, Pinopsida, Gne-topsida, and Angiospermae.
Ginkgoopsida
is just one species; ginkgo or maidenhair tree (Ginkgo biloba).This plant is long
extinct in the wild but is grown on Chinese temple grounds as a decorative
tree. Ginkgo is a large tree bearing distinctive triangle-shaped leaf with
dichotomous venation. This plant is also dioecious (as an exception among
plants, Ginkgo has sexual chromosomes
like birds and mammals) and the pollen is transported by wind to female
(ovulate) trees. The pollen grains of the ginkgo plants produce two
multi-flagellate spermatozoa; the edible seed is fruit-like and becomes ripe
after lying on the ground for a long time. Maiden-hair tree has symbiotic
cyanobacteria in cells. As ginkgo probably went through the population
bottleneck, there are very few, almost no, phytophagous insects that can damage
ginkgo leaves. The only fungus which is capable to eat them, Bartheletia, is also a living fossil.
Cycadopsida—cycads is a class with few genera and about 300
species that growmostly in tropics. Only one species grows naturally in the
United States, Zamiapumila, and can
be found in Florida and Georgia. Cycads are palm-like plantswith large, pinnate
leaves. Their wood is rich of parenchyma since stem has anomalous secondary
thickening. They are all dioecious and its cone is large and protected by
prickles and woody plates. The ovules of these plants are attached to modified
leaves (megasporophylls) that are
gathered in upright cones. Like ginkgo, they have multi-flagellate spermatozoa,
archegonia and large oocyte. Cycad seeds are distributed by animals. Life cycle
is extremely slow.
Pinopsida—conifers are the most widely known and
economically importantamong gymnosperms. Conifers consist of approximately 630
species. Most of them are temperate evergreen trees, but some are deciduous,
such as larch (Larix). The stem has a
large amount of xylem, a small cork, and minute pith. The ovules are attached
to specialized leaves, seed scales,
and are compacted in cones with bract
scales (Fig. 7.16). Some conifers, like
junipers (Juniperus) and yews (Taxus), lack woody cones; these plants
have fleshy scales. Seeds are distributed by wind and animals.
In all, conifer life cycle takes up to two
years. Conifers do not have flagellate spermatozoa; their non-motile male
gametes (spermatia) move inside long, fast-growing pollen tube. Among families
of conifers, Pinaceae (pine family) have resin and needle-like leaves; Pinus have them in shortened shoots, brachy-blasts, and their large cones
have woody scales. Cupressaceae (cypress family)do not have resin, produce
small cones that have a fused bract and seed scales, have dimorphic leaves, and
some of their genera (like “living fossil” Metasequoia
from China) are deciduous in an unusual way: they drop whole branches, not
individual leaves.
Gnetopsida—gnetophytes are sometimes called
chlamydosperms. They are asmall class with only three genera that are not at
all similar: Ephedra, Wel-witschia, and Gnetum. While these plants morphologically remind of
angiosperms,they are molecularly related more to other gymnosperms. Ephedra are horsetail-like desert
leafless shrubs, Gnetum are tropical
trees, and Welwitschia are plants
which have a life form that is really hard to tell.
Both Gnetum
and Welwitschia have vessels (like
angiosperms). Gnetum also has
angiosperm-like leaves with pterodromous venation (however, this probably is a
result of modification of dichotomous venation). Ovules of chlamydosperms are
solitary and covered with an additional outer integument; the male gametes are
spermatia moving inside pollen tube.
Ephedra
has archegonia, but in Gnetum and Welwitschia they
are reduced. On theother hand, Ephedra
and Gnetum have double fertilization: both male nuclei fuse with cells of the one
female gametophyte (endosperm1): with egg cell and another haploid
cell, sister to the egg. Double fertilization in gnetophytes results in two
competing embryos, and only one of them will survive in future seed.
Welwitschia
is probably most outstanding among gnetophytes.
There is only onespecies that occurs in the Namibian desert. The best way to
describe this plant is an “overgrown seedling.” It has a small trunk with two
wide leaves that have parallelodromous venation. The secondary thickening is
anomalous, wood has vessels. Plant is insect-pollinated, and its winged seeds
are dispersed by the wind.
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