Pollination Basics-Part 2: Mechanisms of Pollination
Flowers are important for sexual reproduction by plants. They produce male sex cells and
female sex cells. These must meet for reproduction to begin, a process called pollination.
All living organisms have one major goal in common, which is to pass along their genetic
information to the next generation by creating offspring. Flowering plants create seeds,
which carry the genetic information of the parents and develop into a new plant. In order
for seeds to be created, pollination must occur.
Pollination Vocabulary
| Anther |
The part of the stamen where pollen is produced |
| Cross-pollination |
A type of pollination in which the pollen from the anther of a flower
is transferred to the stigma of a flower of another plant
|
| Dioecious |
Species has male and female flowers on separate plants |
| Endosperm Nucleus |
The nucleus formed by the fusion of two polar nuclei in the embryo sac
of a seed plant prior to fertilization.
|
| Filament |
The stalk that holds the anther and attaches it to the flower. |
| Monoecious |
Species has flowers of both sexes on an individual plant |
| Nectary |
Nectaries are specialized nectar-producing structures of the flower |
| Ovary |
The enlarged basal portion of the pistil where ovules are produced. |
| Petal |
The parts of a flower that are often conspicuously colored |
| Pistil |
The ovule producing part of a flower. The ovary often supports
a long style, topped by a stigma. The mature ovary is a fruit, and
the mature ovule is a seed.
|
| Self-pollination |
A type of pollination in which the pollen from the anther of the
flower is transferred to the stigma of the same flower
|
| Sepal |
The outer parts of the flower (often green and leaf-like) that enclose
a developing bud.
|
| Stamen |
The pollen producing part of a flower, usually with a slender filament
supporting the anther.
|
| Stigma |
The part of the pistil where pollen germinates. |
| Style |
This is the name for the stalk of the pistil. When pollen reaches the
stigma, it begins to grow a tube through the style called a pollen tube,
which will eventually reach the ovary. The style therefore acts as a buffer
against pollen contamination, since only compatible pollen is able to
grow a pollen tube.
|
| Zygote |
A fertilized egg cell that results from the union of male (sperm) and female
(ovum) gametes.
|
Parts of a flower.
It all begins in the flower. Flowering plants have several different parts that are important in
pollination. Flowers have male parts called stamens that produce a sticky powder
called pollen. Flowers also have a female part called the pistil. The top of the pistil
is called the stigma, and is often sticky. Seeds are made at the base of the pistil, in
the ovule.
To be pollinated, pollen must be moved from a stamen to the stigma. When pollen from a plant's
stamen is transferred to that same plant's stigma, it is called self-pollination. When
pollen from a plant's stamen is transferred to a different plant's stigma, it is called cross-pollination.
Cross-pollination produces stronger plants. The plants must be of the same species. For example,
only pollen from a daisy can pollinate another daisy.
It is important to consider that individual plants are bisexual as most plants have
both "male" and "female" parts, situated apart from each other. Based on how a plant reproduced,
this means that auto-pollination (i.e., self-reproduction) is not only likely but inevitable in some
settings.
The male sex cell in a plant, or more specifically the part that bears pollen, is called a stamen and
consists of an anther and a filament.
The female part, which receives pollen grains, is called a pistil and includes an ovary,
a stigma and a style.
Flowering plants contain gametes, which have half of the number of chromosomes in the
plant. These must be combined with other gametes to create a seed with the full number of chromosomes.
The male gametes of a flower are found in the pollen that grows on the stamen, whereas the female
gametes are found in the ovule inside the pistil.
The stamen is made of the anther and the filament. The anther is where the pollen is located at the end of the
stamen. The pistil is made of three parts: The ovary, the style and the stigma. The ovary contains the ovules,
which need to be fertilized in order to produce a seed. The ovules contain an egg nucleus and polar nuclei.
Although many plants grow uni-sexual flowers that have both male and female parts, others grow flowers that
have only a pistil or only a stamen. Some of these species have flowers of both sexes on an individual plant,
known as a monoecious plant, while others only grow one or the other, known as a dioecious
plant.
Dioecious plants must be located near each other in order to reproduce. Hibiscus and lily flowers are uni-sexual
and have both parts. Squash are monoecious, which is why only half of the blossoms will ever grow into
vegetables. Holly is an example of a dioecious plant as it only grows stamen on some plants, while others only
grow pistils.
When the pollen lands on the stigma, the fertilization process begins. The sperm nuclei then travel down
the style through a pollen tube into the ovary, where it enters an ovule.
At this point, one of the sperm nuclei will unite with the egg nucleus and create a zygote. The
other will unite with two polar nuclei to create an endosperm nucleus. The egg and endosperm nucleus grow
inside the fertilized ovule and develop into a seed eventually.
The ovary will then produce a fruit to protect the seed. This could be a fruit protecting a single seed, such as an avocado,
while others have many seeds, such as a kiwi.
A seed is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored
food. The formation of the seed completes the process of reproduction in plants (started with the development
of flowers and pollination), with the embryo developed from the zygote and the seed coat from the
integuments (tough outer protective layer) of the ovule.
A seed develops from an ovule after fertilization. It consists of:
The tesla is the tough, hard, outer coat. The testa protects the seed from fungi,
bacteria and insects. It has to be
split open by the radicle before germination can proceed.
The plumule is the embryonic shoot. In it two or more leaves are usually
visible, with a growing point enclosed
between them.
The radicle is the embryonic root which grows and develops into the root
system of the plant.
The hypocotyl is the part of the stem of an embryo plant beneath the stalks
of the seed leaves
or cotyledons and directly above the root.
The cotyledon is an embryonic leaf in seed-bearing plants, one or more
of which are the first leaves to
appear from a germinating seed.
Feeling confident? Try the pollination quiz:
Pollination Quiz.
Pollination — Part 1, Types of Pollination
Other Pollinaters — Not Just Bees and Butterflies
Do Plants 'See'?
Do Plants 'Breathe'?
The Benefits of Pollen