Upgraded Gymnosperms
Flowering plants
(angiosperms, Angiospermae) are sometimes referred to as “Spermatophyta 2.0.”,
or “upgraded gymnosperms”. In fact, there is no single character which
unequivocally differs flowering plants from other seed plants. Only several
characteristics combined together will distinguish angiosperms. Flowering
plants have their ovules inside an additional cover: pistil which cor-responds with megasporophyll (sporangium-bearing
leaf); later, the pistil de-velops into the fruit.
These plants have an almost complete reduction of game-tophytes: three or even
two cell of the pollen (male gametophyte) and seven (sometimes even four) cells
in embryo sac (female gametophyte),
there are no archegonia or anteridia. Like gnetophytes, they have double
fertilization. The sperms (spermatia) come through the pollen tube (like in
conifers and gneto-phytes). One sperm fertilizes the egg cell, and the other
sperm fertilizes the biggest cell of embryo sac (Fig. 8.1).
While the first
fertilization results in a “normal” diploid zygote which grows into embryo, the
second fertilization ignites the process of feeding tissue develop-ment. This
feeding tissue is endosperm2, frequently triploid
(3n) since it orig-inates from the sperm and cell with two nuclei and sperm, or
diploid (2n), if thebiggest cell of
embryo sac (central cell) had one
nucleus only (Fig. 8.2).
Double fertilization
may be explained in several ways: (1) the second fertil-ization results in
second, “altruistic” embryo which sacrifices itself to feed the “brother”; (2)
second fertilization is only a signal which initiates the develop-ment of
endosperm and it does not really matter which genotype it has; and/or (3) to
make a functional nutrition tissue, angiosperms need a polyploid genome whereas
its origin is not so important.
One way or another,
flowering plants abandoned the pre-fertilization development of the nutrition
tissue, and changed endosperm1 to endosperm2.
In the Mesozoic era,
gymnosperms were the dominating plants of the tree story. However, in the
uderstorey, herbaceous spore plants did not surrender to seed plants and were
still dominating. Amazingly, there were almost no herbaceous gymnosperms! The
explanation is that gymnosperms, being quite advanced in general, had a slow
and ineffective life cycle. Ineffectiveness was in part due to the absence of
sophisticated cross-pollination like insect pollination (which re-quires edible
parts like nectar or excess pollen). In addition, since gymnosperm
fertilization occurs after
gametophyte development, there is frequently a waste of resources: if
fertilization does not occur, then all nutrition tissue (endosperm1) will be lost; such
empty seeds are unfortunately not rare among gymnosperms.
If gymnosperms were to
increase the speed of life cycle, make more sexual struc-tures, grow rapidly,
improve vegetative reproduction, make better pollination and seed dispersal,
they could win the competition with ferns in the under-story.
This is exactly what happened with flowering
plant ancestors. Flowering plants grow fast and restore missing (eaten) parts
with high speed, they parcel-late (clone
from body parts) easily, they have small and numerous floral units(flowers)
which are frequently bisexual but protected from self-pollination and adapted
to insect pollination, they guard ovules with pistil wall, their pollen tube
grows in hours (not days and weeks), they use fruits to distribute seeds.
Fertilization of angiosperms involves the signaling event: when second sperm
fertilizes central cell, it “rings a bell” saying that the first fertilization
is now completed. Endosperm (endosperm2 in that case) will start to
develop only after the fertilization, and resources will not be wasted. This
agile life cycle is the main achievement of angiosperms.
There is a growing evidence that these ancestors
were paleoherbs, herbaceous plants
(and maybe, even water plants like one of the most primitive angiosperms,
fossil Archaefructus, or quite basal
extant Ceratophyllum). Right after
they won a competition with herbaceous spore plants, they started to conquer
the tree storey again, and now, angiosperms dominate the Earth. There are more
than 250,000 species of them which is more than any other group of living
beings ex-cept insects. There are about 300 families and around 40 different
orders. The only places that angiosperms do not grow are the open ocean and the
central Antarctic.
The life cycle of angiosperm (Fig. 8.3) begins much like that of other seed plants;
however, when it reaches the point of fertilization, it changes. The male
game-tophytes, pollen grains, produce pollen tubes which rapidly grow to the
ovule and deeper, to the embryo sac. The embryo sac typically has seven cells
and eight nuclei (two nuclei in the central cell). The first sperm fertilizes
the egg and produces the zygote whereas the second sperm fertilizes the central
cell and produces the mother cell of the endosperm2:
1.
1st sperm
cell (1st spermatium, n) + egg cell (n) - > zygote (2n)
2.
2nd sperm
cell (2nd spermatium, n) + central
cell (2n or sometimes n) - > mother cell of endosperm2
(3n or sometimes 2n)
At the end of life cycle, the flowering plant
develops the fruit (Fig 8.4). Each part of the
fruit is of different origin: fruit skin and wall are from mother plant pistil,
seed coat is from mother plan ovule, endosperm2 is a result of
second fertilization, and embryo is a daughter plant resulting from the first
fertiliza-tion. What is interesting, is that the embryo of angiosperms is still
parasitic: it lives on endosperm which originates from (fertilized, ignited)
cell of female gametophyte—in essence, still similar to mosses.
Angiosperms have four subclasses: Magnoliidae being the most primitive
with flowers of numerous free parts (like water lily, Nymphaea); liliids (Liliidae,
mono-cots) are grasses, palms, true lilies and many others with trimerous
flowers; rosids (Rosidae) with
pentamerous or tetramerous flowers and free petals; and most advanced are
asterids (Asteridae) bearing flowers
with fused petals and reduced number of carpels (Fig. 8.5).
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