Sex in the Vegetable Patch and Elsewhere
How was the sex in your neck of the woods this growing season? Still harvesting those tomatoes, zucchini, and other favorites? Successful sex in your garden and landscape is not always a sure thing. It may take place au naturel after a spring shower, or, perhaps, is assisted by other forces. It may not happen at all because it's too hot, or one of the partners is too stressed. It might not happen for a long time. The conditions have to be right. Everyone has to be ready at the same time. It might even happen over and over again to the delight of all.
Conditions were near ideal this summer, and the reports all around me confirm that vegetable and flower gardens flourished, and fruit and nut trees are very productive. This isn't the case every year, as I'm sure you already know.
I've heard many times the likes of "... there are lots of flowers on my zucchini plants, but there is no fruit developing. What's the deal? I thought they didn't need a partner". I hear the same about tomatoes and peppers. And holly bushes, too. "Why don't my holly plants get any berries?"
The rest of the threads that weave in and out of this tapestry about male and female interaction in the plant world (plant sexual reproduction) seem simple enough. It can also be rather complicated when everything is not just so.
I live for the arrival of sweet corn and tomatoes in summer. If something happens to interfere, I'm mighty upset.
Holly branches with their colorful berries are symbolic at Christmas and New Year and are desirable and joyful decorations for the holidays. We'd like to harvest some of these for that purpose. If we don't after a few years, we are disappointed and concerned. More about this in a little.
First let's visit the sexual side of plant relations. The male organs of flowering and cone bearing plants produce tiny grains called pollen. When pollen is transferred to the female part of a plant, the process is called pollination. When the male sperm cell unites with the female egg cell, the process is called fertilization.
The male flowers contain stamens, the pollen producing structures; these are called staminate flowers. Female flowers contain pistils, the seed producing part that receives the pollen during fertilization, and these are called pistillate flowers.
In most species of flowering plants, nearly ninety percent, the flowers produced are perfect or bisexual, and contain both stamens and pistils (at the base of it all, sperm and egg).
In some species, staminate and pistillate (male and female) flowers are on the same plant. Such species are known as monoecious species. Corn, apples, plums, tomatoes, cucumbers, begonias, lilacs and pines are some of the many.
The remaining plants, only about six percent, are dioecious like the holly mentioned above, as well as poplars, willows, maples, spruces, asparagus and others.
There are two common methods of pollination: cross-pollination and self-pollination. Cross-pollination involves the transfer of pollen from a stamen on one plant to a pistil on another plant. In self-pollination, pollen is transferred from a stamen of one flower to a pistil of the same flower or to a pistil of another flower on the same plant. Self-pollination (the actual transfer) and fertilization (the union of sperm and egg) is impossible in dioecious species because the male and female flowers are on different plants.
Cross-pollination is the method of pollination in most flowering plants. The method requires an agent to carry the pollen from flower to flower. Insects are the most common agents of cross-pollination. Wind is another. The wind pollinates some kinds of flowers, especially those with very light pollen that have small, plain blossoms. Insects or birds pollinate most plants that have showy or sweet-smelling flowers. Glands that produce nectar near the base of the flower serve as a powerful food attractant.
Apples and peaches, for example, both need pollinators and if it is cold or wet or there are other reasons to keep the bees (severely lacking, as they are) and other pollinators from doing the job, fruit set will be poor.
The flowers produced on holly plants are imperfect. This means that the flowers are either male or female on each individual plant. A close up examination will reveal which is which. Yet, the function of flowers is a reproductive one, that is, to make seeds. Since both sexes will never appear on the same plant, a landscape with only male plants in the area will not produce holly berries. A landscape with many females and one male will produce an abundance on each female plant, unless some other conditions exist to cancel this out.
Cross-pollination requires helpers such as honeybees and moths to carry the pollen from the male holly plant to the stigma of the female plant and the magic happens. If it is too cold or too wet there may not be any flying helpers to carry the process to a further stage. Sometimes, all looks right but the plants themselves may be stressed from cold, heat, drought, improper planting, wrong soil pH, and on and on. Consequence: no fruit. See what I mean when I say it may be complicated.
Plants in the Cucurbit family, squashes, cucumbers and melons, have incomplete flowers and are monecious. Each plant contains both male and female flowers. Only when males and females are flowering at the same time will successful fruit set take place. Frequently male flowers will appear first, sometimes for two or more weeks before female flowers appear. The unfertile, male flowers will wilt and die. Sometimes female flowers will appear first and appear to set fruit without pollination, but this fruit also will wither and die. There are summers where the zucchini and cukes never stop until it gets cold.
And, to complicate things even a bit more, there are parthenocarpic cucumbers: these plants provide only female flowers that set and develop seedless fruit with no pollination required.
There is yet another type of plant with imperfect flowers, though examples of it are rare. This type, gynoecious, fits some cucumbers. Gynoecious plants are normally monoecious but under some conditions may produce a tremendous number of female flowers, with only a few (if any) male flowers. Since only the female flowers produce fruit, gynoecious plants can produce very high yields.
Honey bees and other bees, moths and butterflies, and some beetles and flies feed on nectar and pollen. In traveling from flower to flower seeking food, pollen grains are inevitably transferred and flowers become cross-pollinated.
So, the next time you venture out into your garden and yard, go armed with a small paintbrush or two. If fruit set seems below par and the season is getting on, you can assist nature and transfer some pollen to the pistils of waiting flowers. This is especially important for plants that produce heavy pollen grains such as cucumbers and their cousins. Wind can't assist. Insects or you are the agents needed.
The Ginkgo tree is known as a living fossil and is truly a beautiful tree with wonderful fan-shaped foliage. It is dioecious. The fruit is foul smelling, as rancid butter. Plant only males if you want to enjoy this specimen tree. Some females are desirable (holly, bayberry and pyracantha) and some are not so (Ginkgo, mulberry, Osage orange).
I earnestly hope that with this better understanding of the sex life of flowers, you'll know why some of your asparagus plants get berries and some don't. The beauty of some ash trees is in the brilliant clusters of berries they hold. Females, right? Don't forget to plant a male, too.
From The Garden of Ed. Submitted for publication in The Towne Crier on September 26, 2007
© 2007 Ed Mues. All Rights Reserved.