Have you ever been curious about the sex life of a dahlia? It is a miracle. The miracle
is accomplished by very simple parts. Those of us who grow dahlias have all seen
the parts that make up the male and female components of a dahlia. We may not have
known what they were called or how they work.
Those of us who grow dahlias from seed get very attached to the seedlings. They become
parts of our families. You may hear dahlia growers call their new varieties their
babies or their kids. Some growers just name the new flowers after their family members
or friends. Others give them their own family name or give them their own name and
make them one of their family. For those of us who are so attached to our kids we
might want to know more about how the miracle happens.
The end result is a seed. The seed is a start of the next process. But that is another
story. I will start with the end of the story. The seed is formed in the flower.
The first picture is a seed head. The seed head is from Edna C. How to get seed from
Edna C, but that is another story.
The seed head is what is left after the flower dies and the petals fall off or are
taken off. As is seen in the picture, seeds form across the whole seed head, not
just at the central disk. Each petal that forms mature male and female parts can
generate a seed. I will use pictures of the various parts and the definitions from
the encyclopedia to let you guide yourself through the process. The pictures are
in magnification of 10 , 60 and 200 magnification.
Flower, specialized part of seed plants that contains reproductive organs. The basic
floral parts (sepal, petal, stamen, and pistil) are modified leaves, typically arranged
concentrically and attached at their bases to the tip of the stem. The outermost,
green sepals (the calyx) encircle a whorl of usually showy, colored petals (the corolla),
within which POLLEN-bearing stamens surround a central ovary-bearing pistil. After
fertilization, each ovule (the part that contains the egg) in the ovary becomes
a SEED, and the ovary becomes the FRUIT. The number and arrangement of floral parts
varies greatly among groups of plants and are important bases for classification.
In general, the higher a plant is on the evolutionary scale, the greater the flower's
complexity and efficiency for reproduction. It consists of four kinds of modified
leaves, two of which (stamens and carpels, the latter sometimes called pistils)
bear pollen and seeds. Several flowering plants also produce pollen and seeds on
STRUCTURE OF FLOWERS
Four kinds of modified leaves make up a complete flower: carpels and stamens (primary
reproductive structures) and petals and sepals (secondary structures). The carpel
is the female reproductive structure. It has a stigma, where the pollen becomes attached
and germinates; a style, through which the pollen tube grows; and an ovary with one
or more ovules. The egg cell that will unite with the sperm cell (delivered by the
pollen tube) forms in the ovule. The stamen is the male structure; its filament supports
an anther, in which the pollen is formed. The often brightly colored petals are important
in attracting pollinators, and the often leaflike sepals enclose the bud before the
flower opens. The many pieces of flowering plants are usually distinguished from
one another by the way these four basic flower parts are modified, although closely
related species within a genus may have quite similar flowers.
In some major groups, pollen is transferred by the wind. The products of the flower
are the seed and the fruit. The seed is the mature ovule. It includes a minute embryo
plant and, almost always, stored food that will supply the seedling when it begins
to grow after sprouting, or germination.
POLLINATION is the transfer of pollen grains in seed plants from their production
site in pollen sacs on male structures to a receptive female site on the same or
a different plant. Generally Cross-pollination--pollination between two plants--may
only occur in plants of the same species.
A pollen grain is a partially developed male gametophyte, the sperm cell of the plant.
Optimally, when the pollen grain reaches the female site, it will produce a pollen
tube, which grows inside to carry the sperm cells close to the female reproductive
cells. Pollen tube transport of sperm cells in seed plants permits fertilization
without free water, which is required by all other groups of vascular plants. This
feature has allowed the seed plants to occupy a wide variety of terrestrial environments,
including deserts. It is important to note that pollination does not always result
During pollination, individual pollen grains are in a dry, inactive state. Once deposited
on female sites, they are moistened and activated. The emergence and growth of a
pollen tube from a pollen grain requires metabolic activity and consumption of starch.
The pollen grain and the female site must also be compatible for pollen tube development
to occur. Compatibility depends on the maturity of both pollen and female site and/or
chemical interactions between the two.
Wind pollination, nearly universal among gymnosperms, is a specialized development
in flowering plants. It is thought to have evolved in plants that invaded cooler,
drier environments where insect numbers were more limited than in the tropics. Wind
pollination, because of its relative inefficiency, requires a large number of pollen
grains per ovule.
Corolla (ke-rol e, -ro’le) noun The petals of a flower considered as a group or a
pis·til (p¹s"t…l) n. The female, ovule-bearing organ of a flower, including the
stigma, style, and ovary.
Stigma Botany The receptive apex of the pistil of a flower, on which pollen
is deposited at pollination.
Style Botany The usually slender part of a pistil, situated between the
ovary and the stigma.
Ovule Botany A minute structure in seed plants, containing the embryo sac and
surrounded by the nucellus, that develops into a seed after fertilization.
Pollen (pòl´en) noun The fine, powderlike material consisting of pollen grains
that is produced by the anthers of seed plants.
se·pal (s"p…l) n. One of the separate, usually green parts forming the calyx
of a flower.
Now you know the basics. You may want to try your hand at hand polinating. With these
definitions the process may make more sense to you.
Gametophte n. Botany 1. The gamete-producing phase in a
plant characterized by alternation of generations. ga·me " to·phyt
Definitions excerpted from The American Heritage® Dictionary of the English Language,
and Grolier Encyclopedia®.
Photos by Steve Boley
Seed in a seed pod that has been cut in half
It takes only one grain of pollen to pollinate the stigma.