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Understanding How Flowers Are Pollinated

Understand more than the basic process by which plants make seeds. It's very helpful for gardeners to learn more about the physiological processes leading to seed production.
By Robert Gough and Cheryl Moore-Gough
January 19, 2012
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Attracting bees to your garden will improve pollination of many types of plants, but sometimes you'll need to protect certain plants from accidental cross-pollination by bees or other insects.
Photo courtesy Storey Publishing (c)2011


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Excerpted from The Complete Guide to Saving Seeds(c) by Robert Gough & Cheryl Moore-Gough, with permission from Storey Publishing. The following excerpt can be found on Pages 11, 19, 20, 21 and 22.  

Saving seeds is a fascinating and delightful pastime. Seeds are sometimes the “poor stepchild” in the garden, overlooked by gardeners in their pursuit of a bountiful harvest of fruits and vegetables. But we’ve discovered that collecting seeds has been a satisfying extension of our other gardening efforts, and being seed savers has made us better gardeners overall.

Plants do a fine job of producing their own seeds year after year, but when we save seeds for replanting, we must manage the process so that we end up with the healthiest, most vigorous, and most reliable seeds possible. Once our plants set seed, we must know how to collect and store those seeds so their seedlings grow well the next year.

Most gardeners know the basic process by which plants make seeds, but it’s very helpful to learn more about the physiological processes leading to seed production. Among these are how plants produce flowers, how flowers are pollinated, a bit about plant breeding, and how seeds form and ripen. Along the way, we’ll familiarize you with some of the terminology that botanists use to distinguish different types of fruits and seeds. So put on your student cap, and we’ll get started right away with how flowers are pollinated and the process of pollination.

How Flowers Are Pollinated

Pollination is the transfer of pollen from stamens to pistils or from staminate strobili to ovulate strobili. Without pollination, there can be no union of the male and female gametes, and therefore fertilization of the ovule cannot occur. If fertilization does not occur, there will be no fruit and, thus, no seeds.

A self-pollinated plant is one that is pollinated by its own pollen due to flower structure or isolation (physical separation from the pollen of other varieties or species of plants). Cross-pollinated plants are those that are pollinated by pollen from other varieties of the same kind of plant or, in some cases, by other species of plants in the same genus.

Seed savers need to be mindful of the sources of pollen that their seed-stock plants are exposed to because of the potential for different varieties of a plant to cross-pollinate. Cross-pollination can lead to unpredictable results because it “mixes up” the genetic composition of the seeds. For example, pollen from a red cabbage plant carries the genes for red leaf color. If pollen from a red cabbage plant lands on the pistil of a green cabbage plant, those red color genes will become intermixed with the green color genes in the seeds that form. If you plant those seeds, you will end up with “off-type” plants. Most will still have green leaves, but they will not necessarily look like the green cabbage parent.

Plant breeders have come up with a variety of methods for preventing unwanted cross-pollination. Some of these methods aren’t practical for home gardeners, but others work very well. We explain the nuts and bolts of these methods, which are referred to as isolation techniques, in chapter 2 of The Complete Guide to Saving Seeds.

How Pollen Gets Around 

Pollen is dispersed from one flower to another mainly by wind (botanists call this anemophily) and by insects (entomophily). Wind pollination is common in conifers and in some angiosperms that lack showy flowers, such as spinach, corn, and hickory trees. It’s the most common type of pollination.

Species with brightly colored and/or scented flowers, such as roses, squash, and apple, often have heavy or sticky pollen grains that cannot float on the wind. These plants are insect-pollinated, primarily by bees. Quite a few shrubs and understory plants such as rhododendron depend on insect pollination because the overstory plants block free wind movement. These plants would have died out long ago were it not for insects. Some species, notably some maples, willow, and mulberry, are pollinated both by wind and insects.

There are plants that are self-pollinated; these require no assistance from wind, insects, or other critters to set fruit. Tomatoes and legumes, such as beans and peas, fall into this category. Cross-pollination may still occur in these plants, though, due to opportunistic insects seeking pollen or to a brief gust of wind that blows pollen from one flower to another. For instance, a bee may happen to visit a tomato blossom in your garden, pick up pollen, and transfer it to another variety of tomato. After all, the bee doesn’t know that those flowers are supposed to be self-pollinated! In the long run, it doesn’t matter much to nature; the mission has been accomplished. But it will matter to you, the seed saver, because of your wish to maintain genetic purity of the seeds produced.

A few plants are pollinated by bats (chiropterphily), wasps, butterflies, and other agents. Magnolia, a primitive genus, is pollinated by beetles. Yucca is pollinated by moths; the toad cactus (Stapelia), which smells like rotted flesh, by flies; and the saguaro cactus (Carnegiea) by bats. Water pollination (hydrophily) is rare but occurs in eelgrass (Vallisneria). Almost all flowers pollinated by birds (ornithophily) are red.

Fertilization 

When pollen lands on a stigma (which is moist and sticky when receptive), the pollen grain germinates, and a pollen tube grows down the style and into the ovary, releasing, in most cases, two sperm cells. One sperm cell unites with the egg in the ovule to form the embryo, and the other unites with two polar bodies to form the endosperm, a tissue that will nurture the embryo while it’s in the seed, just as an egg yolk nurtures an unborn chick. In this case, the flower undergoes double fertilization.

Pros and Cons of Cross-Pollination 

Natural cross-pollination increases genetic diversity, vigor, fruitfulness, and pest resistance, but it also can result in the loss of desirable traits you want to perpetuate. Controlled, or intentional, cross-pollination by plant breeders (or by you, the home garden seed saver) can result in plants that have more desirable traits.

Most older varieties of vegetables and flowers are open-pollinated, which means that they are pollinated by some natural means, whether it’s wind or insects or bats. They are different from F1 hybrid varieties, which have been created by very specific and controlled cross-pollination between two parent plants that have particular characteristics.

Open-pollinated plants will produce seeds that are reasonably true to type only if planted in isolation, and this is a key point for seed savers. Let’s study the example of spinach, which is a naturally cross-pollinated species. ‘Bloomsdale Long Standing’ is an open-pollinated variety of spinach. Commercial seed growers grow ‘Bloomsdale Long Standing’ in fields that are isolated from all other varieties of spinach. That way they can be sure that their female plants are being pollinated only by pollen from males of the same variety. Thus, the genetic traits incorporated into the seeds produced will be within the acceptable known characteristics of ‘Bloomsdale Long Standing’.

If you plant only ‘Bloomsdale Long Standing’ spinach in your garden, you can be reasonably sure that the seed you save from your plants will produce a ‘Bloomsdale Long Standing’–type plant from year to year. However, if you plant different varieties of spinach in your garden in the same year and allow them both to bloom, they will cross-pollinate. Let’s say, for example, that you planted not only ‘Bloomsdale Long Standing’, which has crinkly dark green leaves, but also ‘Lombardia’, which has smooth leaves that are not so deeply green. The seeds that result from cross-pollination of these varieties will carry a combination of traits from the two. Of course, if you allow only one of the varieties to flower, no cross-pollination can occur. Now, what if your neighbor grows spinach, too? Can you still save seeds from your spinach plants? You’ll need to visit your neighbor’s garden and check on the spinach plants. If none is in flower, there can be no cross-pollination. You’re safe!

Isolating open-pollinated plants may be difficult or impossible if you are collecting seed in the wild. For example, you won’t find a single oak in the woods, but instead a stand of oaks that likely represent a wide variety of genetic traits. Since they’ll cross-pollinate, the acorns you save cannot reproduce precise replicas of the parent.

Hybrids and Pollination 

From a horticulturist’s perspective, a hybrid plant is one that results from a cross between two inbred lines (plant lines that have been produced by a series of self-pollinations). Scientists control this process very closely; that is, they carefully choose two parent plants with certain characteristics and pollinate one with the other. Many varieties of vegetables and flowers, and some varieties of woody ornamentals, are what are known as F1 hybrids. The designation F1 means the “first filial generation,” or the first generation of a plant after a controlled cross was made.

In the wild, though, hybridization takes place easily between naturally cross-pollinated species, but since we don’t know to which generation a particular wild plant belongs and what the parent plants were, we don’t call them F1 hybrids. The results from this type of natural hybridization are much less predictable than those from controlled hybridization.

F1 hybrids usually produce vigorous, high-yielding, pest-resistant plants with high-quality flowers, fruits, or roots. Or they may have been bred to produce fruit that ripens early, that will store longer, or that has redder fruit, more colorful foliage, disease resistance, or any number of other characteristics the breeder sought.

F1 hybrid seeds produce plants true to type, but if you save seeds from an F1 hybrid plant, the plants that sprout and grow from those seeds won’t be, because they won’t have the precise genetic mix that results from the original controlled cross. Such plants are called the F2 generation. Many F2 plants are likely to have fairly desirable qualities as well, but as you continue to save seeds, those qualities may be lost. Most likely, if you keep on saving the seeds, you’ll wind up with plants far inferior to the original F1 parents. If you want to experiment with saving seeds from hybrids or with making your own hybrid crosses, you can learn about that in chapter 7 of The Complete Guide to Saving Seeds.

It can be lots of fun to experiment with saving seeds from the natural hybridization that’s happening in your yard. Willows, for instance, are wind-pollinated, and pollen from many different willow trees may blow onto the stigmas of your tree, there to unite their genetic material with that of your tree. The result will be seeds that will produce plants with variable characteristics. Nature provides you with a variety of choices, as though to say, “Here are multiple seedlings with many characteristics. Choose what you want, and propagate it.” If you want to reproduce a plant that is exactly like the parent, then take cuttings or try some other method of asexual propagation. But if you like the idea of exploring the potential for surprise, work with the seeds.

Buy The Complete Guide to Saving Seeds 


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