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The birds and the bees of plant reproduction

by Timber Press on May 19, 2017

in Gardening, Natural History

A well-defined pistil sits at the heart of each grapefruit flower. After fertilization of the eggs, most of the flower parts drop off, leaving the green ovary nestled in a tiny cup formed by the sepals. Photos by the author.

One of the special rewards of day-to-day work in a garden is witnessing the unfolding reproductive cycles of plants. Here are some interesting insights into the ways plants proliferate: when a stamen and a pistil love each other very much…

Pollination by Animals

Having been attracted to a flower by its color, shape, or aroma, an insect or other small animal may become an agent for pollination (pollen transfer). In some species, particular visitors are the only means by which pollination can be accomplished. Poised on flexible filaments, the anthers dust pollen on the animal’s body; as the little courier makes its rounds to other blossoms, some of its pollen load is brushed on their stigmas. For such a method to be successful, however, both the anthers and stigma must be strategically positioned to make contact with the animal as it probes the flower.

Differences between species’ floral designs, including overall shape and the exact placement of stamens and pistils, are the products of natural selection. Many flowers are precisely engineered to match the body forms of the animals participating in pollen transfer. Because of their shape, some flowers are exclusively pollinated by a single species of bee, wasp, or fly—a precarious dependency as extinction of the insect would leave little hope for the plant species’ survival.

Pollen transfer by animals is a more rapid, direct, and certain process than its random dispersal by wind or flowing water. Honey bees and hummingbirds, for example, move quickly between flowers, thereby dispersing pollen before the flowers wilt and stigmas become unreceptive. And, it would seem, even the smallest insects possess the desire to fly consistently between flowers of the same species—the most important requirement for successful pollination.

Sexual reproduction can only occur between plants of the same species; foreign pollen landing on a stigma is incapable of delivering sperm to the eggs. It is believed that a flower obtains clues to the alien pollen’s incompatibility from the grain’s shape and chemical composition.

Pollination Alternatives

The far-reaching effects of sexual reproduction lie in the selective advantages of hybridization, genetic mixing that occurs when gametes from different parents of the same species unite. In many species of flowering plants, elaborate methods have evolved favoring cross-pollination and consequent cross-breeding (outbreeding). These include self-incompatibility, chemical barriers in the stigma that treat a plant’s own pollen as if it were from another species; spatial separation of the anthers and stigmas in a bisexual flower; or staggered timing of pollen release and the stigma’s receptiveness in each flower.

Cross-pollination is also ensured when separate male (staminate) and female (pistillate) flowers are formed either on the same or different plants. When one plant bears both types of flowers, it is called a monoecious condition (Greek: mono, “one”; oikos, “household”). Monoecious species include corn (Zea mays), walnut (Juglans spp.), filbert (Corylus spp.), melons (Cucumis spp.), and squash (Cucurbita spp.). In dioecious (“two household”) species the two flower types are borne by separate individuals. Examples include willow (Salix spp.), date palm (Phoenix spp.), and pistachio (Pistachia vera).

Male flowers of begonia bear a cluster of yellow stamens.

In begonia’s female flowers, the pistil’s yellow stigmas and styles are connected to an ovary, hidden below the perianth.

So important is pollen transfer that some species possess self-pollinating backup systems for use when cross-pollination fails to occur, such as on cool days when insects are not active. In such cases, the flower’s own pollen is not treated as a foreign strain. Although self-pollination precludes genetic diversity through hybridization, as a last resort it is preferable to no reproduction at all.

Some mechanisms of self-pollination include anthers that eventually sweep past the stigma as the stamen’s filaments slowly curl, nasturtium (Tropaeolum majus) being an example. In other species, the filaments may elongate and carry the anthers past overhanging stigmas, or movement of the style may bring the stigma into contact with the anthers. In foxglove (Digitalis spp.) the stamens are attached to the bell-shaped corolla. As the corolla is being shed at the end of the flowering period, the anthers may touch the pistil and thereby transfer pollen.

Although the mature flowers of many species open in a variety of environmental conditions, a few remain closed and undergo self-pollination in response to cold temperatures or certain photoperiods, a condition called cleistogamy (literally, a “closed marriage”). Such plants, to which insects are not attracted, conserve energy by not having to produce metabolically expensive nectar during periods of poor growth. The flowers of some species of rock rose (Cistus spp.) and salvia, for example, remain closed in cold climates, whereas several species of violet (Viola spp.) develop open flowers in spring that are insect pollinated and closed flowers during the long days of summer. Many flowers remain closed at night and on rainy or overcast days, simply to protect the pollen against moisture.

Self-pollination does not necessarily occur during those periods. Flowers that open after the sun has set, however, are prepared for pollination by nocturnal animals. Such flowers have pale colors, visible in dim light, and most emit their strongest fragrances during the hours of darkness. On a midsummer’s evening in a garden where honeysuckle or night-blooming jasmines grow, one can share the ecstasy of nocturnal moths as, lured by sweet scents saturating the air, they are irresistibly drawn to the waiting blossoms.

The Reproductive Process

The stage is set for reproduction when, by one means or another, compatible pollen comes to rest on a flower’s stigma. Of the two cells within a pollen grain, one is destined to grow into a long tube, a pollen tube, that penetrates the pistil’s tissues in search of a microscopic opening in one of the ovules, located in the ovary. Germination and growth of the pollen tube is rapid and is promoted by food substances and hormones supplied by the stigma and style. Botanists still do not understand how a pollen tube locates the ovule’s tiny pore or the mysterious factor that diverts several tubes, growing from many pollen on the stigma, to separate ovules.

The second of a pollen grain’s cells divides to become two sperm that move through the pollen tube and enter the ovule. Before the pollen tube and sperm’s arrival, each ovule must be equipped with an egg, ready for immediate fertilization. A zygote is formed when one of the two sperm unites with the egg. The second sperm combines with another cell in the ovule. The product of that union is a temporary food-storage tissue, called the endosperm, used to nourish the zygote as it grows into an embryo, the miniature plant within a seed. A portion of the endosperm may persist in a seed and supply food to the growing seedling during seed germination.

As ovules grow and mature into seeds, they remain enclosed by the ovary, which slowly enlarges to become a fruit—the angeion (Greek: “vessel or container”) part of the word angiosperm, the flowering plants’ formal name. (Sperma is Greek for “seed.”) The development of most fruits and seeds occurs as a consequence of pollination and fertilization of the eggs. Embryo and seed growth apparently stimulate the production of hormones, including gibberellin, that promote fruit enlargement. When pollen tubes deliver sperm to the ovules in only one segment of the pistil, the resulting fruit merely enlarges on that side. Gardeners are familiar with such occasional, odd-shaped products.

 

Brian Capon received a PhD in botany from the University of Chicago and was a professor of botany at California State University, Los Angeles for thirty years.

 

 

 

 

 

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