Seed savers need to learn a little about how plants produce seeds and how they are classified. With this knowledge, you will find that you make fewer mistakes when trying to grow plants for seeds. It isn’t necessary to know a great deal about the science of botany, however, and the fundamentals are fairly easy to understand.

The Sex Life of Plants

Plants produce seeds by sexual reproduction. All flowering plants will have male and female parts, either on the same plant or different plants of the same species. The male parts are called stamens, and include a pollen producing sac called an anther and a filament to support it. The female part is called a pistil, composed of the ovary at the base, a tube coming out of the ovary called a style, and a pollen-receptor called a stigma, on the top of the style. These parts can look very different in different types of plants; for example in corn, each strand of silk is the stigma and style, while the male parts are at the top of the plant, and called the tassel.

Tulip flower showing six anthers and three stigmas.

Pollen produced in the anthers is transferred by various ways, depending on the type of plant, to the stigma, which is often sticky. When fertile pollen hits the stigma, it sends down a pollen tube to the waiting ovary and fertilization is accomplished. Then a seed begins to form.

Pollination depends on wind for grasses (grains), several kinds of nuts, corn, and other plants whose flowers are drab and unobtrusive. Plants with brightly coloured flowers are pollinated by insects, birds, and other animals. A few types of plants, like tomatoes and beans, are self-pollinating.

Sunflower being pollinated. The “flower” of a sunflower is actually composed of many tiny flowers called florets. In this photo, each of the yellow dots around the centre of the head is a separate flower, with male and female parts. The “petals” around the head are also separate, but sterile, flowers.

Many flowers are “perfect”, with male and female parts in the same flower; examples are tomatoes and cherry blossoms. Other plants have male flowers and female flowers on the same plant, but these look different (“imperfect” flowers); squash, cucumber and melons are examples. Still other plants have male flowers on one plant and female flowers on a different plant; spinach and asparagus are examples.

Cross-pollination

A hybrid plant results from pollen from one variety getting on the stigma of another variety, either deliberately or accidentally. Anyone saving seeds will have to learn which plants cross-pollinate easily (hybridize), which ones may cross depending on circumstances, and which ones generally do not because they are self-pollinating. If you want to have seeds whose progeny look and taste like the parent plants, cross-pollination must be avoided. We will get into details of these plants in future blogs.

“Open-pollinated” means that seeds you harvest will produce vegetables that are like the ones on the parent plant. Many open-pollinated varieties are heirloom or heritage types. These have been passed down from generation to generation, especially in European countries, and were generally of limited availability until recently. But not all open-pollinated varieties are heirlooms.

Hybrid fruits and vegetables are perfectly fine to grow if you don’t care about saving seeds. Most commercial hybrid fruit and vegetables were developed to provide better disease resistance, uniformity, vigour, and consistent high yields. By developing named hybrid varieties, plant breeders can control intellectual property rights for their seeds.

Hybrid seeds are produced commercially either by hand methods or genetic methods. The hand-pollinated method is labour-intensive, and involves removing the male part of each flower and transferring pollen from a different variety. This technique is largely done in Asian countries, Chile and Mexico. Most of the hybrid seeds of tomato, peppers, and melons are derived from hand-pollination. The genetic-controlled hybrids are derived from plants that are bred to be self-incompatible or have male sterility. This method is largely done in developed countries, and includes brassicas, onions, carrots, and beets. These are not genetically engineered plants, but ones developed from selective breeding.

Plant Classification

One of the best ways to prevent cross-pollination is to understand which plants are related to the other plants you are growing. Plants that are not related to each other will not hybridize.

Plants are classified according to their relationships with other plants. A large grouping of related plants is called a Family. There are eight major vegetable families, plus a few others not widely grown. Grasses and most herbs have different families.

As an example, the family Brassicaceae contains familiar plants like cabbage, radish, and kale. Members of this family are broken down into smaller units, called genera (singular genus). For example, cabbage, broccoli, kale, and turnip all belong to the genus Brassica, while radish belongs to the genus Raphanus. Then, under each genus are even more closely related types, called species. So, for Brassica, there is for example, Brassica oleracea (Broccoli) and Brassica rapa (Chinese cabbage).

You will be able to learn quickly which plants might be related, just by looking at them. Onions, tomatoes, beets, bok choy, and corn all have different leaf structures and growth habits. They are in different families of plants. Plants in one family will never cross-pollinate with plants in another family, while plants in the same genus might, although unlikely. And of course, varieties within the same species will hybridize easily.

In another blog, we will discuss how many plants of each species should be grown for good seed production, as well as the distance various species should be separated to make sure your seeds “come true”, that is, produce vegetables that are like the parent plant.

Resources
Ashworth, Suzanne. 2002. Seed to Seed, 2nd Edition. Seed Savers Exchange, Decorah, Iowa, USA.
Campbell, N.A., J.B. Reese, and L.G. Mitchell. 1999. Biology. Addison Wesley Longman, Inc., Menlo Park, CA.
Tay, David. Vegetable Hybrid Seed Production: http://www.seedconsortium.org/PUC/pdf%20files/23-vegetable%20Hybrid%20Seed%20Production.pdf