Asexual reproduction is cloning. A piece of a plant
may root or sprout and grow into a new plant which is genetically
identical with the parent.
- Sexual reproduction results
in a new plant which contains genetic material from two parents, but which is not genetically
identical with either one. These new plants may be
able to adapt more successfully to environmental changes than
their parents could.
and multiply on land, plants needed to evolve new structures and
methods for reproduction. An important advance was the
development of the flower, a
structure which allowed widespread scattering of its pollen and
yet provided a stable, nurturing environment in the ovary for
developing seeds. Pollen could be carried from flower to
flower by insects, the wind, or, sometimes, birds.
The seed was
another important innovation, as significant for plants as the
amniotic egg was for land animals. Seeds would form when the
chromosomes of the parents were united in the ovary. A seed is
a compact package made of a cell capable of growing into a plant,
food to help the plant get started, and, in angiosperms, a seed coat
that protected the seed from dehydration and damage in
the environment. Properly protected seeds could survive until
conditions favored their development. In the deserts today,
some seeds may lie dormant for many years until the rains come and
provide conditions in which the seeds may
germinate and grow into plants.
Some seeds also are contained in, or attached to, structures which
help the seeds to be carried to
other places that may be suitable for them.
plants are able to reproduce asexually. Some plants send up shoots
from their roots and form big clumps of stalks and leaves. Some
can grow leaves from pieces of root.
Many of our house plants
can grow from a leaf or a short piece of stem.
plants store food in underground tubers and then grow new plants
from the tubers: potatoes do that. Some plants grow from bulbs
which can grow small new bulbs at the root level.
Some plants can grow from cut
off leaves or stems. Many of our house plants are shared this
way. Some plants even grow little plants on their leaves or root
when a piece of stem is buried.
All these methods of
reproduction work well. However, they do not make it possible for
plants to move to new locations. They result in plants with the same
characteristics as the parents: the same resistance to the same
diseases, the same responses to flood, drought, heat and cold, and
the same schedules of growing. They are vulnerable to everything
that might destroy the parents. A viable community needs members
with diverse strengths and vulnerabilities.
In addition to reproducing asexually,
most land plants also reproduce sexually.
Once plants got out of the water
and onto the land, they faced real challenges. One of those problems
was to invent ways to share and to scatter genetic material.
Some of the early plants, ferns, for instance, found ways to
exchange reproductive material in water, and later released many
tiny spores into the air. However, much of the earth is too dry
for this strategy. Plants had to find ways to deal with the dry
A group of plants called gymnosperms
developed wind borne pollen. These were trees -- cycads, ginkos
and needle-bearing trees such as pines and redwoods. Tall plants
get more wind than those close to the ground, and the gymnosperms
developed small male pollen-bearing cones. Their pollen was released
into the air and drifted to other trees in the forest. Today,
if you are in a pine forest in spring, you can often see a golden
haze of pollen grains in the air.
The plants also developed female
cones which are essentially ovaries. The pollen falls directly
on the female cones, and the pollen grains grow tiny tubes into
the ovary to find the chromosomes and join with them. The female
cone grows into the cones that we are familiar with, and the
seeds are tucked safely between the bracts.
Gymnosperm means naked seeds. These
seeds have only a very thin covering that probably does not
offer them much protection.
Wind pollination seems rather
extravagant, as surely most of the pollen grains never find a
female cone. However, the system works, and has worked for millions
of years. Gymnosperms grow in forests and groves, where the tall
individual plants grow close together, so there are many potential
targets for the pollen grains.
The Development of Flowers
A new group of plants, the angiosperms,
appeared about 110 million years ago. These plants had developed
a number of structural innovations, the most striking of which
is the flower. Flowers enchant us with
their beauty, delicacy, and variety of form, but they represent
a very practical development. Plants, by growing flowers and
fruit, formed partnerships with animals who provided transportation
for pollen and seeds.
flowering plants still use wind to transfer their pollen to other
plants. The grasses, growing thickly together in meadows and
on plains, continue to rely on wind pollination.
Many flowering plants, however,
use insects or birds to distribute their pollen. Insects can
be lured to the flowers by a few drops of sweet nectar. Brightly
colored petals guide the insects toward the nectar and pollen.
Bees can fly for several miles in a day, and, if all members
of a plant species come into bloom at about the same time, the
bees spread their pollen far and wide. Pollen sticks to the hairy
bodies and legs of insects, and is easily carried away. Many
of the fruits that we eat are dependent on insect pollination.
Another invention of the Angiosperms is the development of the seed
coat on their seeds. Each seed is enclosed in a tough
little covering to help it to survive in the world until conditions
favor successful germination
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