GardenLine | Porpourri - Miscellaneous | The Sex Life of Plants

Wood was an instructor with the Department of Horticulture Science. This column is offered as an extension service by the Division of Extension and Community Relations and the Department of Horticulture Science, University of Saskatchewan.

Here at GardenLine we often receive calls from puzzled gardeners wondering why their cucumbers are not producing fruit. People also call us in droves about lack of fruit production on such plants as native grapes, buffalo-berries and sea buckthorns. To understand why plants sometimes go fruitless, you need to know something about plant sexual reproduction, the subject of today's column.

Back to the Basics

Most of us learned something about the basics of plant sexual reproduction in public school. In a flower with both male and female organs (a "perfect" flower), the male portion of the flower produces pollen and the female portion produces the eggs. The male organ, the stamen, consists of anther and filament; the female organ, the pistil, consists of stigma and ovary. When the pollen is transferred to the stigma and successfully fertilizes an egg in the ovary, a viable seed is produced. So much for the basics.

Beyond the Basics

What complicates matters somewhat is that there are in fact 3 categories of flowers:

1. The perfect flower with fertile male and female organs both on the same flower. Plants with flowers in this category include apples, plums, lilacs, potentilla, tomato and many ornamental flowers;

2. The perfect flower with male and female organs both on the same flower, but with either the male or the female organ infertile. In this category are included plants in the sunflower family and many other hybrid ornamentals. These ornamentals usually have large, showy flowers that are "double" or "semi-double" - with more than the normal number of petals.

3. The imperfect flower with either fertile male or fertile female organs, but not both, on the same flower. Spruce, pines, ash and maples have imperfect flowers.

Going Farther

To complete the picture, we have to add another complication. Plants with imperfect flowers are either monoecious or dioecious. Let's make things a little easier for ourselves by calling a flower with only male organs simply a male flower, and one with only female organs a female flower. Then, monoecious plants have both male and female flowers on the same plant. That is, if you go to a particular monoecious plant you will find male flowers and you will also find female flowers. In contrast, dioecious plants have male and female flowers on separate plants. If you look at a particular dionecious plant, you will find either male flower or female flowers but not both.

Corn, cucumber and other vines are monoecious; ash, maples, buffalo-berry, some junipers and aspargus are dioecious.

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.

What it All Means

Knowing something about plant sexual reprodcution, we can now answer some of those puzzling questions:

Question: Why is my green ash loaded with seeds, when my neighbor's green ash has no seeds at all?

Answer. Well, ash trees are dioecious, right? That means you likely have a tree with only female flowers, and so they produce plenty of seeds; you neighbor's green ash likely has only male flowers and hence no seeds - because, of course, male flowers don't produce seeds.

Question: Why do some of my asparagus plants produce red berries, while others in my vegetable garden produce no berries at all?

Answer: Ah, what you have are asparagus plants, which are dioecious. That means some of the plants have only female flowers, and these are the plants that produce the berries. Other plants have only male flowers, and male flowers cannot produce fruit; hence no berries there.

Question: I have some buffalo-berry plants that have never produced fruit. Why?

Answer: The buffalo-berry is dioecious. Your particular plants likely have flowers of all the same sex. That is, if you have 3 plants, they either all have male flowers or they all have female flowers. If they all have male flowers, expect no fruit of course. If they all have female flowers, again expect no fruit because the female flowers have to be pollinated by male flowers if fruit is to form.

The obvious solution would be to plant a buffalo-berry of the opposite sex nearby. Unfortunately, in practice things are not that easy. You cannot determine the sex of a plant until it starts to flower and fruit - unless the plant was "asexually" propagated (say by cuttings) and then the sex of the offspring will be the same sex as the original plant. Probably the most practical approach is to obtain another buffalo-berry and plant it nearby. Then you'll have a fifty-fifty chance of success.

Question: Why are my cucumbers not producing any fruit? I read you column today and I know that cucumbers are normally monoecious. You say that means any particular plant with have both male and female plants. The male plants will be fertile. The female plants will be fertile too. So what's the problem? Why no fruit?

Answer: One subject I should perhaps have discussed earlier is how the pollen gets from the male flower to the female flower. Pollen on plants such as corn is quite light and is easily transported by the wind to the female flowers. In contrast, cucumber pollen is to heavy to be wind-borne and is usually carried by insects such as bees. So, if some of your cucumber plants did not produce fruit, it is could be that there was a shortage of bees or other pollen-carrying insects in the vicinity of those plants

One solution would be to hand-pollinate the female flowers on your cucumber plants next season. Simply remove a male flower and touch its anther to the stigmas of the female flowers present. One male flower produces enough pollen to fertilize a number of female flowers.

One further point, however. There is a possibility that your problem may be caused by a condition that is not related to fertilization. Temperature or drought stress on cucumbers may cause the plants to form only male flowers; hence no fruit will be produced.

Question: How do you account for missing kernels on corn in my garden? Some of the cobs have kernels missing here and there. On other cobs, many rows of kernels are absent.

Answer: Corn is monoecious remember. Each cob is actually a flower that contains many female organs. . Above the cobs are the tassles, which are the male flowers. Pollen from the tassles is carried by the wind to the female organs, each of which will produce a single seed, or kernel, when fertilized. If kernels are absent the likely explanation is that pollen did not reach some female organs, probably because the wind carried the pollen away from the cob. Planting corn in squares rather than long rows will help to assure pollination.

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Sustainable horticultural information, offered free of charge to the public with the support of the University of Saskatchewan Extension Division, the Department of Plant Sciences and the Provincial Government.