BSCI 124 Lecture Notes
Undergraduate Program in Plant Biology, University of Maryland
LECTURE 21 - POLLINATION
I. Sexual reproduction and evolution
A. Sexual reproduction aids in evolution because sexual reproduction produces
variable offspring upon which natural selection works
B. Sexual reproduction is advantageous only if an organism mates with someone
other than itself (outcrossing or outbreeding)
1. Animals- most species have separate male and female individuals
2. Potential problem for plants- most
both male and female parts in them (perfect flowers)
3. Many flowering plants have evolved special ways to assure outcrossing
and prevent self-crossing
4. Pollination, the transfer of pollen from the male anther to the female
stigma, must occur before seed plants can reproduce sexually.
A primary function of flowers is to attract pollinators with colorful petals,
scent, nectar, and pollen.
II. Pollination & fertilization
summary; strongly recommended for review
A. Pollen contains a sperm nucleus and a tube nucleus. There is no
B. The s perm nucleus is enclosed and prote cted by the gametophyte tissue.
Since the sperm is protected, fertilization is no longer restricted to damp
environments. The protected sperm can blow through the air. The male and
female gametophytes can live anywhere as long as the sperm can reach the
egg. As a result, the gametophyte generation of seed plants became drastically
reduced in size and is attached and dependent on the sporophyte.
C. For a protected pollen sperm (generative cell) to successfully fertilize
a protected egg, there must be a method to get the pollen from the male to
the female. This process is called pollination.
D. The male gametophyte (pollen) travels from the microsporangium to the
megasporangium containing the female gametophyte with the egg. The pollen
grain sticks to the receptive (sticky) part of the megasporangium.
E. The process of fertilization begins when a second cell within the pollen
grain, a tube cell, creates a tube through the megasporangium to the egg.
F. When the egg has matured, the pollin's generative cell travels down the
tube where it divides to produce sperm to fertilize the egg. In most gymnosperms,
this can be a two year process.
G. The gametes are never really exposed. The sperm develops within a pollen
tube and is liberated at the archegonium. The egg is continuously surrounded
by the gametophyte tissue. The covering around each megasporangium further
s the protection, not only to the megagametophytes and eggs but also to the
III. How do plants get pollen from one plant to another? (plants are
rooted in the ground)
IV. Coevolution - the interactions between two different species affect
the development of each other's characteristics during the course of evolution
The pollen structure is often related to its method of pollination.
A. In insect-pollinated species, the pollen is frequently sticky or barbed.
B. Wind-pollinated species usually produce lightweight, small, smooth
C. Pollen grains have a limited life span. Depending on the species, pollen
may be viable for only a few hours up to several weeks after it is released
from the anther.
A. Bees - Bees are the most important group of flower pollinators
1. The bees live on the nectar and feed it to their larvae. With special
mouth parts, hairs, and other body adaptations, they are especially suited
to collect and carry these materials.
2. Bees are guided by sight and smell.
a. Bees see yellow and blue colors. They also see ultraviolet light as a
distinct color and many flowers have ultraviolet markings. Bees are not able
to see the color red, so they rarely pollinate red flowers.
b. Some flowers have "honey guides" that guide bees to the nectar. They often
have a "landing platform".
c. The nectary is usually situated at the base of the petals. The pollen
is normally large and sticky.
B. Butterflies - Like bees, butterflies are guided by sight and smell.
1. Some butterflies are able to see red as a distinct color, so some
butterfly-pollinated flowers are red and orange.
2. Butterfly or moth-pollinated flowers are often in the shape of a long
tube because each insect has a long proboscis.
3. The pollen is normally large and sticky.
C. Moths - Most moths come out only at night, so the typical moth-pollinated
flower is white or pale with a strong sweet odor that often is only emitted
1. The pollen is normally large and sticky.
D. Flies - Some flowers look and smell like dung or rotten meat. Dung and
carrion flies mistakenly lay their eggs on the flower and in the process
collect and pass pollen. When the eggs hatch, the maggots starve for lack
E. Birds - Birds have a good sense of color and are attracted to colorful
red or yellow flowers. They do not have a keen sense of smell, so bird-pollinated
flowers usually have little odor.
1. Birds are attracted to red and yellow flowers.
2. Pollen does not readily stick to the bill, but does to the feathers.
3. Since birds lap up the nectar with their tongue, bird-pollinated flowers
produce nectar that is more fluid and in greater quantity than insect-pollinated
4. Hummingbird-pollinated flowers usually have a long, tubular corolla suited
to their long beak.
5. The pollen is normally large and sticky.
F. Beetles - Beetle-pollinated plants are often dull in color or white, but
produce strong odors.
1. Bats are usually active at night, so flowers they pollinate are often
2. One mouse-pol linated flower has inconspicuous green blossoms that open
at night, with a well-hidden source of copious, sucrose-rich nectar. When
the rodent grasps the flower and inserts its tongue to probe for nectar,
the pollen is released explosively from the anthers, dusting the rodent's
H. Wind - The flowers are small in comparison to animal-pollinated flowers.
1. The pollen grains are light, small, and are not sticky.
2. The stamens are exposed to the wind so that the pollen can easily blow
in the wind.
3. The stigmas are feathery and exposed to the wind so that they can catch
4. The flowers are often grouped in inflorescences to facilitate pollen release
VII. Why animal pollinators "service" flowering plants
Other Sites of Interest:
Yucca and yucca
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Last revised: 30 Jan 1997 - Delwiche