What's So Special About Flowers?

Natural History, May, 1999 by Karl Niklas

Fertilization is so nice, they do it twice.

Flowers. We see them every day--in our gardens, homes, offices, and art museums. A perennial source of inspiration for poets and artists as well as fashion designers and wallpaper manufacturers, flowers nourish the human spirit. They mark nearly all the significant events in our lives, from birthdays to funerals, and they are powerful symbols of the human spirit--from the red rose of romantic love to the sacred lotus of Buddhism. On a more immediate level, flowers feed us every day with the many delicious fruits--and seeds--of their reproductive labors.

Flowering plants, or angiosperms, made their evolutionary debut more than 140 million years ago and went on to become the most successful and diverse group of plants in the history of the earth. Numerous traits contributed to their spectacular rise to dominance, including the ability to reach reproductive maturity rapidly. But at the heart of what defines a flowering plant is a special way of producing viable, nutritious seeds--an evolutionary achievement that was to prove nearly as important to the success and global spread of human beings as it was to the angiosperms themselves.

A flower forms when a specialized stem stops growing and produces a set of highly modified leaflike structures. Some of these structures (the petals) serve to attract pollinators; the function of others (the sepals) is to protect the body of the flower, although natural selection has drafted some of these into the colorful business of attracting pollinators as well. Still other structures--no longer bearing any resemblance to a leaf--form either the male part of the flower (the stamens, each consisting of a filament and a pollen-containing anther) or the female part (the carpels, each consisting of a stigma, which receives the pollen; a thin stalk, or style; and, at the bottom of the style, an ovary). The flowers of some angiosperms possess all four types of leaflike structures. The cherry blossom is a textbook example of a so-called perfect flower, with petals, sepals, stamens, and carpels all clearly visible. Wind-pollinated grass flowers, by contrast, which don't need to attract pollinators, have no obvious petals, while magnolias and many other flowering plants lack clearly defined sepals. But all reproductively functional flowers bear either stamens or carpels, or both.

The business of stamens and carpels is sex. Sperm cells develop inside the pollen; egg cells develop within ovules inside the ovaries. The "goal" of nearly all flowering plants is to bring the sperm and egg together to produce an embryo, which then develops inside the seed until it germinates. Then the new plant can begin making its own food--through photosynthesis--and, ultimately, its own flowers.

Functionally, angiosperm flowers differ not a jot from the cones of pine trees, which belong to a group of seed-producing plants collectively known as gymnosperms. Both flowers and pine cones house the plant's sex organs (pines produce two kinds of cones: one that makes pollen and one that makes seeds). In addition, both nourish their embryos as they develop inside the seed. But flowering plants keep their ovules completely hidden from view, safe inside the carpels, while the pine ovule must be exposed to the air to receive pollen. The very word angiosperm derives from the Greek for "vessel" and "seed," whereas gymnosperm comes from the Greek for "naked" and "seed."

The real distinction between the angiosperms and the gymnosperms, however, has nothing to do with whether the seeds are hidden or exposed to view. The sine qua non of angiospermy is a phenomenon called double fertilization. After landing on a carpel's receptive stigma--whether blown there by the wind or deposited by a pollinator--a grain of pollen sprouts a long tube that grows down through the style until it reaches an ovule. Once inside the ovule, the pollen tube delivers two sperm cells. One ultimately enters the ovule's egg cell, where the nuclei of the two cells fuse. This fertilized cell then divides repeatedly, producing an embryo.

Meanwhile, the second sperm cell delivers its nucleus to another cell in the ovule, known as the central cell. Typically much larger than the egg cell, the central cell has two nuclei, which fuse with the nucleus of the second sperm. The result of this union is the production of a tissue that is unique to flowering plants: the endosperm, a metabolic go-between that collects and stores nutrients from the parent plant early in the development of the seed and later imparts these nutrients to the growing embryo. (Some flowering plants have lost the ability to make endosperm; the seeds of many orchid species, for example, are no larger than a speck of dust and contain little or no endosperm.)

Endosperm, which is rich in fats, proteins, and carbohydrates, prepares embryos for germination and early survival. It has contributed to the ability of angiosperms to produce a mind-boggling diversity of species and to invade nearly every nook and cranny on the planet. Humans--by eating the seeds of corn, rice, and wheat before they have had a chance to germinate--have also been major beneficiaries of the wealth of nutrients stored in the endosperm. In addition, most seeds go through a period of dormancy, during which their tissues dehydrate and lose weight, allowing them to germinate when conditions are favorable and--of vital importance to human cultures around the world--permitting the storage of large quantities of comparatively lightweight food for long periods of time. Double fertilization may not be the stuff that artists and poets dream of, and it may never be foremost in the thoughts of gardeners whiling away a dreary winter day with the colorful pages of a flower catalog, but human civilization as we know it would not be possible without it.