There is a very good chance that the
vast majority of the plants you can see out your window are angiosperms.
They have taken over the land since their rise during the
Age of Dinosaurs, pushing the other groups to the ecological
fringes. Gymnosperms still have an advantage in systems that
are too cold and/or dry (they become more common with higher
altitude and higher latitude), or have nutrient-poor soils, and so
haven't been completely pushed out of business.
Reproduction in angiosperms
follows a course similar to that in conifers,
with the gametophytes confined to a periodically-made part on
the main sporophyte. These gametophytes are parts of
the characteristic feature of the group: flowers.
Although some species have separate male and female flowers, most
have both male and female gametophytes in each flower.
Many angiosperm flowers don't look like what you might
expect of a flower - this is discussed below in the section
on coevolution.
Angiosperms get their name from
the fact that the seeds (which are sporophyte embryos packaged with
some fuel to sprout with) are produced inside a fruit,
a structure used to move seeds away from the parent plant. All
angiosperms produce fruit, although some might not be things you
would see as fruit: fruits can be built to fly through the air
(maple trees and
dandelions make this type),
float across the water
(coconuts), stick to passers-by (burrs), as well as be eaten by
animals. Even edible fruit is almost never meant to act as a
food source for the seeds inside.
Angiosperms are vascular
plants, containing xylem (going up) and phloem
(bringing down) in various
bundled patterns. They have true roots, for holding the
plants firm as well as for taking in water and nutrients; they
have true stems, for support and for moving materials up and
down; they have leaves, usually flat, for most of their
photosynthesis. Leaves, and
sometimes stems, have pores called stomates that can
be opened or closed. When open, carbon dioxide for
photosynthesis enters air spaces inside the leaf through which it
gets to photosynthetic cells, but water is also lost (this is the
main source of transpiration from angiosperms). By
controlling the pores, a plant can get enough carbon dioxide for its
needs and minimize its water losses. Clusters of xylem and
phloem, plus a reinforcing sheath to help support the leaf, are
called veins.
Angiosperm stems may be green (herbaceous),
which are capable of photosynthesis or woody. Green
stems have support limits; woody stems, with ring after ring
of tightly-packed cells with very rigid cell walls, are present in
the large angiosperms, the trees. Gymnosperms also
produce trees with ringed stems.
As was true in the gymnosperms, the "main
plants" in angiosperms are sporophytes, while the gametophytes
are confined to the flowers, usually male and female together. Male gametophytes are called
stamens,
female gametophytes are pistils. The pollen
from the stamen has to reach the pistil and make a pollen tube
to the base of the flower, the ovary, and the ovules,
where the egg cells are. Once the egg cells are fertilized,
the embryos are sealed up with food in a seed and the ovary is
converted into a fruit.
The seeds often wait for some environmental cue, such as warmth,
moisture, or light periods shifts, before germinating,sprouting. A type of growth hormone known as auxins,
which settled to the bottom of the sprouting root and stem as they
emerge, have different effects on those parts: auxin-soaked
root cells grow more slowly, making the top of the root grow faster
and curve the root downward; auxin-soaked stem cells grow
faster, curving the stem up.
In a
mature plant, auxins migrate away from the sunlit side of the plant
- if the light is coming in from the side, the migration causes the
stem to curve toward the light as it grows and better orient the
leaves to catch light. Plants produce many different
types of hormones that can affect reproduction, overall
growth, defenses, fruit ripening, and other features.
Production
of new cells in plants happens in a type of tissue called the meristem.
Meristems can be, and usually are, at the growing tips of the
plants, where they are called apical
meristems.
Most plants add new cells from the tips out, not evenly all over and not from
the bottom up - in the other parts of the plant, the cells
grow but do not divide. Leaves, branching stems or
roots, and flowers
all are produced by apical meristems.
Some growth may also occur along the sides of the
plants, such as is found in the
rings of trees - these are called lateral
meristems. These make
tree ring patterns in ecosystems with
growing seasons, where growth produces big,
"light" cells, alternating with seasons of less growth
(cold winters or dry periods), which produce smaller,
"darker" cells - each light/dark zone is a ring, and the
wider, lighter the ring, the better that growth season, leaving a
record of year-to-year climate.
THE
TWO MAJOR SUBGROUPS OF ANGIOSPERMS
The flowering plants are
generally treated as two separate groups, named for a
feature in their
seeds called a cotyledon. The
monocots have a single-piece seed (corn
is a good example of a monocot), while the dicots
have a two-piece seed (a peanut is a clear example of
the seed, but a maple may be a better overall example
of a dicot). There are
several general
differences
between the two subgroups:
Root
Systems mostly branch out equally from base (Fibrous
Roots)
Root
Systems mostly have one major root from which smaller
ones grow (Taproot)
Stems
do not grow in Ring Pattern (never woody)
Stems
sometimes grow in ring pattern (sometimes woody)
Vascular
Bundlesform ring pattern in roots, scattered
in stems
Vascular
Bundles in middle of root, form ring in stems
There are a few other differences between the
groups, including subtle differences in pollen, but the listed ones
are the best known and easiest to recognize. There are
many exceptions, so usually a combination of traits must be
looked at to be sure which group a plant belongs to - and
even then, some plants are not so clearly in one group or the
other.
Vascular
bundles are collections of xylem,phloem,
and other tissues,
laid out in different patterns in the two groups. In
woody plants, the outermost rings are the active xylem,
with active phloem on the underside of the bark.
Peeling a strip of bark from all the way around a tree will
cut off the roots' food supply (phloem, remember, carries
fuel from photosynthesis down to the lower parts) and
eventually kill the tree.
Since a significant part of any organism's ecosystem
is the other organisms it shares that system with, it should come as no
surprise that much of evolution is driven by interactions between
individuals of different species, sometimes driving long-term
relationships between the species. The interactions can be
cooperative, as happens in a symbiosis,
or more of an ongoing battle, as happens between predator and prey
or plant-eater and plant. The evolutionary process where two
species adapt to each other over time is called coevolution.
Coevolution
is obvious throughout the angiosperms. The most well-known is
the use of animals as pollinators, carriers of pollen
from the male parts of a flower to the female parts of another
flower. These angiosperms have evolved ways to make their
flowers stand out in
the environment to their pollinators, both visually and with smells,
and often present "rewards" such as food to bring
pollinators back over and over. The construction of a flower
is often a clue to its pollinators. Many flowers without
pollinators have no
colors or odors, and pollen bearing and catching parts stick out in
the air: these are wind-pollinated, common in many
grasses. Some flowers are broad, with stumpy parts:
these are often pollinated by crawling insects. Some flowers
are tall and narrow, with a central platform: these are
commonly pollinated by flying insects, who press past the anthers on
the way in and pick up pollen, then land on the next flower's
pistil, leaving pollen there. Flowers can give clues about
their pollinators: the reason that red flowers rarely use bees
(and often use beetles) as pollinators was a clue toward the
discovery that bees don't see red as a color (although they can see
ultraviolet as a color, and some bee-pollinated flowers are
ultraviolet-colored). Fruits also have often evolved to use
particular types of animals as seed-carriers, although the
relationships are rarely as carrier-specific as those that have developed
with pollinators.
Other
types of coeveolution exist in angiosperms: many defenses,
such as thorns and poisons, are
set against particular types of enemies.
Many of the drugs that humans use recreationally are derived from
the poisons plants have developed to fight their insect
enemies: the active ingredients in tobacco, cocaine, and
marijuana are all neurotoxins, natural equivalents of RaidTM.
One of
the lesser-known theories about the decline of the dinosaurs
hypothesizes that the plants that served as the basis of their food
chain adapted defenses against them and effectively put the
"out of business." It seems unlikely, especially
given that our plant-eating relatives haven't been fought off very
well.
Informational
Links
Way
more than you ever wanted to know about
auxins.
How to
get many different seed varieties
to germinate.
KEY CONCEPTS - Click on term to go to it in the
text.
Terms are in the order they appear.
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