Let’s face it, when you’re confined to a hole in the ground, finding romance can be challenging. To compensate, plants have perfected many showy, smelly, sticky and sly ways to mate. Don’t let the apparent lack of sexual differentiation fool you.
Flowering plants have male parts and female parts, sperm and ovaries. Over the millennia, plants have developed methods that allow them to pair up just fine, with a little help from nature. Sometimes the process is a community effort; other times individual plants have all the necessary equipment to perform the honors without interference or much fanfare.
Of course, the goal is the continued existence of a particular plant variety, either intact or with minor variations designed to act as insurance—a hedge against future environmental upset. There’s usually plenty of drama in the process, too. Let’s explore the mechanics of plant pollination—a steamy theme if ever there was one. Don’t worry; this story has a happy ending.
What are the different parts of a flower called?
To get the general idea, it’s useful to view flowers as factories designed for reproduction, made up of male and female parts with names like: stamen, pistil (not pistol), anther and stigma. Sure, flowers are so compelling and appealing that it seems counterintuitive to see them as functional rather than decorative.
That’s the trick, though. When you wake up and smell the roses—and view flowers with an objective eye—it’s easy to recognize there’s more going on than the poets have let on. Flowering plants produce the equivalent of eggs and sperm in either an open or closed system. Here’s how it works:
On the female side, several rows of cells called ovules within the ovary of a flower’s hollow pistil, or female sex organ, undergo a process called meiosis, dividing and duplicating their chromosomes to create haploid spore cells and eventually female gametes and mature sexual reproductive (egg) cells. This is science speak for getting ready to receive plant sperm and develop seeds.
On the male side, cells in the anther (male sex organ) also undergo meiosis, producing microspores which develop further into pollen grains that contain sperm nuclei encased in a protective outer coating. These grains are typically sticky and distributed along the exterior of the anther portion of the stamen.
Pollination is the process that brings pollen and plant sperm, or male DNA, in contact with plant eggs, or female DNA, an essential step in the production of seeds. This is also where things start to get interesting.
Imagine a plant in full bloom, all ready to get hooked up and see what nature has in store. Now what? You can see there’s a potentially enormous problem. It’s not like plants can uproot themselves and attend singles’ night at the local bee and brew. They are well and truly stuck—in a hole—for the duration of their lives. Without some creative chicanery, ripe and ready plants aren’t going to produce many plant seeds or baby plants.
You might argue that a plant could just go ahead and pollinate itself. That’s actually one classic method of plant reproduction. Either a single flower will fertilize itself, or the honors will be performed by the pollen from another flower on the same plant. Self-pollination can be a closed system or act as a kind of fall-back pollination method if cross-pollination doesn’t occur. Self-pollination is quite efficient, but the resulting plants will be less genetically robust than their cross-pollinated counterparts. Why? They lack genetic variety because they derive from a single source.
In a perfect world, this wouldn’t matter too much, but when environmental conditions change in the real world, limited genetic variation reduces the chances of survival—not just of one plant, but of an entire species. Variety is one of nature’s premier survival strategies, and skipping that process isn’t a good idea. That’s why some flowers have safeguards in place specifically designed to avoid self-pollination.
Scientists believe there are around 400,000 flowering plant species in the world. At least 75% of those species rely on cross-pollination as a means of producing viable seeds. To aid in the cross-pollination effort, nature has come up with ways to give plants an assist. Think of it as pollen special delivery from one plant to another:
Wind — It is widely believed that most cross-pollination occurs as a result of the action of insects and animals, but wind does play a role. Ask any allergy sufferer, and he’ll tell you that plants producing wind-borne pollen compensate for their generalized and somewhat inefficient delivery strategy by producing massive quantities of pollen. Wind pollination, or anemophily, is often associated with trees, like conifers, and plants that have exposed ovules that are easy to access without insect assistance. It’s interesting to note that as a special adaptation, the pollen most often dispersed by the wind is also usually somewhat lighter in weight than pollen typically transported by insects, birds and animals.
Wings — It seems as though most plants prefer a winged assist when it comes to pollination, favoring the action of bees, birds, bats and other winged creatures that can really make the rounds from flower to flower. The process goes something like this: fliers looking for tasty pollen or nectar are attracted to the fragrance, shape or bright coloration of a particular flower, land and leave with a freight of sticky pollen, which they deposit in the next flower, and the next and the next. Pollen grains enter the stigmas of subsequent flowers, while the fliers pick up additional pollens, visit more flowers and make more deposits.
Over time, some of these delivery methods have become more specialized to accommodate the most likely pollinators for a specific plant in a particular region. For example, white flowers are often preferred by insects active during the evening or overnight hours. Where these insects are abundant, white or light yellow flowers are likely to be more abundant, too. This is the principle of symbiosis in action.
Although fliers are common pollinators, crawling and hopping insects and small mammals can be pollinators, too. Even water can play a role in pollination. We stress fliers here because bees are big players in the pollination effort. Data released by the US Department of Agriculture suggests that around 75% of the vegetables, fruits and nuts produced annually are pollinated by bees. This, if nothing else, makes them a workforce deserving of respect and protection. Remember, pollination is essential for the production of seeds and the development of new plants.
A Note About Plant Duplication
Cross-pollination is exactly what it sounds like. It mixes the genetic characteristics of two different plants. This blending won’t affect the parent plants, but will have an impact on the seeds they produce. Sometimes new crosses result in more attractive, flavorful, pest-resistant or drought-hardy offspring. Sometimes crosses result in less favorable characteristics like smaller fruits or less showy blooms. For an exact duplicate of a plant, cloning and grafting are among the most common methods of vegetative propagation.
For example, the genetic origins of the first Hass avocado tree are unknown, but were likely the result of cross-pollination—a happy accident as it turns out. Since the discovery of this creamy, flavorful avocado variety, though, whole orchards of Hass avocado trees have been grafted from the exact genetic stock provided by that single original tree. It’s a guacamole success story, and also an homage to nature’s ability to produce favorable crosses. Once those plant crosses exist, though, preserving them through cloning and grafting makes a lot of sense.
Pollination and Hydroponics
Pollinating plants manually makes it possible to direct the course of future generations by selectively determining which plants to cross. If you want to develop a four-pound tomato, a vibrant new orchid or a pumpkin the size of a freight car, dabbling in genetic manipulation may or may not help you achieve your goals. For the average indoor gardener, though, pollinating flowering plants by hand is usually a necessary chore. Unaided, cross-pollination indoors can be an iffy proposition. With a couple of tools and good timing, however, it’s easy for a helpful gardener to step in and play cupid.
When performing the honors, you’ll likely be dealing with two basic types of flowers: distinct male or female, or mixed male/female. Distinct male and female flowers have a specific gender. Once identified, pollination involves moving pollen from a male flower’s anther to a female flower’s pistil. Some distinct gender varieties have flowers of both genders on a single plant. This makes pollinating them somewhat easier. Other varieties have all-male or all-female flowers on individual plants. Propagating them requires having at least one flowering male and one flowering female plant available.
Mixed male and female flowers contain both male and female sex organs. In nature, a little shake is all that’s usually necessary to pollinate many of them, although you may want to swap pollen among different specimens.
Pollinating Plants By Hand
Hand-pollinating flowering plants is a pretty straightforward process. I’ve hit the highlights below:
- Identify blooming flowers in the morning with an eye toward pollinating them early in the day.
- Determine which type of flower, distinct or mixed, you’re dealing with.
- On each flower, identify the anther portion of the stamen that contains pollen (male), and the pistil structure (female) where the pollen will be deposited. If you’re having trouble envisioning the location of these organs for a specific plant variety, refer to a botanical illustration for reference. Don’t worry too much. It’s pretty intuitive.
- For distinct male and female flowers, use a cotton swab or soft-bristle craft brush to gather pollen from the anther of the male flower and brush it on the pistil of the female flower. Female plants will often have small fruits attached to their flowers, making them easy to identify.
- Be sure to use clean, sanitary brushes or swabs.
- Manual pollination may require more than one treatment. You may also want to make a note of the date and the specific plants you are treating.
- For mixed male/female flowers, give each flower a gentle shake to help transfer the pollen from the anther to the pistil. Repeat the process a few times to be on the safe side. Some self-pollinating plants like beans require no assistance. A little research will tell you which varieties need help and which don’t.
Just for the fun of it, try hand-pollinating a few blossoms in the garden this spring. It’s a good way to get a bird’s eye view of things. It’s also great practice for any manual pollinating duties you plan to carry out later in the season.