Dahlia Reproduction

By: Sandy & Steve Boley

 

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 modified leaves.

 

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 in fertilization.

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 unit.

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

 

 

 

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Seed in a seed pod that has been cut in half
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It takes only one grain of pollen
to pollinate the stigma.
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