Fortunately, a Ph.D. in botany or genetics isn't necessary to grow plants successfully from seed. But this chapter on seed basics will help you better understand what seeds are, where they come from, their role in plant reproduction, and why they are so critical to the health and survival of plant species. It will also help you appreciate—the next time you head for the pumpkin patch, perennial r, fields, or woods—the wonders of these typically tiny parcels of life, which contain all the fundamental parts of a mature plant—leaves, stem, and root.
A complete, or perfect, flower contains both a female part—the pistil, usually composed of a stigma, style and ovary—and male parts—stamens, usually composed of a filament and an anther. An incomplete flower is either male or female.
And Then There Were Seeds
Plants didn't always produce seeds. Millions of years ago, when the world was mostly water, swamp, or just plain wet, terrestrial spore-producing plants like ferns and mosses were supreme because they used water to facilitate reproduction. Once the continents began to drift apart and land rose, the earth's climate began to fluctuate. The seasons were born. Seed-bearing plants were one adaptation to these drier conditions. Scientists divide these higher plants into two groups: the gymnosperms (gymno, meaning naked, and sperm, meaning seed), such as pines, firs, and cycads; and the angiosperms (angio, meaning contained in a vessel), or flowering plants. Seed-bearing plants, which are of most interest to the majority of gardeners, represent one of the most important steps in the evolution of the plant kingdom.
The life cycle of a gymnosperm is clearly represented by the white pine, Pinus strobus. Every spring, pollen-bearing male cones appear at the ends of the tree's lower branches, clustered just below the new crop of needles. When mature, the cones release clouds of pollen, which are carried by the air to the female cones growing at the trees' tops. After being fertilized by the pollen, the individual eggs mature into embryos. That process takes a minimum of 13 months, not including an additional year or so for the seeds to fully develop. Finally, mature and winged, the seeds gently glide to the ground, where they will lie through winter, waiting for spring's warmth to prompt them to germinate.
About 75 percent of all the seed-bearing plants on earth today are not gymnosperms, but rather angiosperms. All angiosperms produce flowers, which botanists define as shoots, modified for reproduction." Although gardeners value flowers for their shapes, colors, and fragrance, nature designed them solely as a means of reproduction. As a result, most flowers have similar elements.
First, the typical flower has a receptacle, a structure that holds the rest of the floral parts together. Before opening, the flower petals are protected by a calyx, an outermost whorl, or ring, made up of modified leaves called sepals. A second inner whorl, called the corolla, is made up of petals. Collectively, the calyx and corolla are known as the perianth of a flower.
Petals are often brightly colored in order to attract insects and birds (a few flowers, such as the poinsettia, appear to have brightly colored petals that actually are modified leaves called bracts; in a handful of flowers, such as tulips, these colorful parts are called tepals, because it's difficult to determine if they are sepals or petals). Petals can serve as both beacons and landing fields for insects, and many petals even have lines on the surface that act as arrows, leading pollinators to the flower's center.
Inside the whorl of petals are the stamens, the male parts of the flower (see illustration, above). The typical stamen consists of a long stalk, or filament, with a swollen tip called the anther, which is the structure that produces grains of pollen. The female part of the flower, known as the pistil, usually includes the stigma, the sticky portion that traps pollen; the style, the stemlike portion that holds the stigma where it can best catch pollen; and the ovary, the swollen base of the pistil. Inside the ovary are immature seeds, or ovules, waiting to be fertilized.
When flowers have some form of all these parts, they're known as complete. Complete flowers are also known as perfect flowers, because they contain both male and female parts; imperfect flowers have either male or female parts, either stamens or pistil, but not both. Plants, such as sweet corn, squash, and cucumbers, that contain both male and female flowers are called monoecious. When the male and female flowers occur on different plants, as in hollies, asparagus, and persimmons, the plants are called dioecious; to produce flowers—and seeds—you need to have both a male and a female plant in your garden, or at least in the immediate vicinity.
Seed pods come in many different shapes and sizes, such as: false indigo, Baptisia australis (A); wisteria, Wisteria sinensis (B); husk tomato, Physalis alkekengi (C); poppy, Papaver somniferum (D); mamane, Sophora chrysophylla (E); and flax, Linum perenne (F).
Flowers usually must be pollinated for seeds to form. There are two basic kinds of pollination: self-pollination and cross-pollination. Self-pollination occurs when the pollen of a flower fertilizes that same flower or another flower on the same plant. The typical self-pollinating species has perfect flowers, the ones with both male and female parts. Peas, lettuce, tomatoes, snap beans, and snapdragons are examples of self-pollinating plants; they can be grown from seed without fear of crossings that may result in plants with unwanted variations from the parents.
Cross-pollination results when pollen from one flower fertilizes a flower on another plant. The flowers of cross-pollinating plants can be either perfect or imperfect. The constant mixing of genes that occurs with cross-pollination is crucial in helping species remain healthy and vigorous.
After centuries of observation, we know that cross-pollination is carried out naturally by several types of pollinators. The most important pollinators are the wind, insects, birds, bats, and, finally, water. Wind-pollinated plants normally have no nectar, no fragrance, and no brilliant colors to attract wildlife. Instead, their floral structure is suited to sending and receiving pollen on the breeze. Grasses are a good example; although their flowers are visible, they are tiny.
Plants that are pollinated by insects, birds, bats, and other wildlife tend to have bright, attention-grabbing flowers—real advertisements for themselves. The payoff for the pollinating animals is food: either pollen, which is eaten by some insects, or nectar. Many flowers are specifically designed to accommodate the animals that will pollinate them. The foot-long spurs of the Christmas star orchid depend on a specific moth with a foot-long tongue; the stink of carrion flowers attracts the tiny flies that transport its pollen; flowers pollinated by bats tend to open at dusk, the same time that bats become active.
We humans are also adept at a specific type of cross-pollination, known as hand-pollination. The objective of hand-pollination is to avoid random cross-breeding—and thus to guarantee that the seeds you save will produce plants like their parents. To insure this, you must prevent insects from visiting the flowers of plants you've selected for seed saving. Then you must perform the insects' job yourself.
On a microscopic level, pollen can vary greatly from plant to plant: willow (A), dandelion (B), apple (C), ruellia (D), mimulus (E), pine (F), chervil (G), passiflora (H), robinia (I), rhododendron (J), buckwheat (K), acacia (L), geranium (M), and sunflower (N).
Sex To Seeds
Imagine it's a bright sunny day, somewhere in the temperate zone. The sun is high in a blue sky and a breeze moves across a field of waving grasses and brightly colored wildflowers. You stop at a clump of sundrops, a day-flowering species in the evening primrose family. On top of a two-foot stem, bright yellow, four-petaled flowers are in full bloom. From stage right, enter a bumble bee, his hind legs bright yellow from pollen. The bee buzzes in flight, then spots the flowers. It swoops down and sticks its tongue into the four-pronged stigma at the flower's center. As the bee moves about in a somewhat clumsy way, pollen grains attach themselves to the stigma.
What happens next takes place on the microscopic level. When a grain of pollen becomes attached to the stigma, one of its two cells stimulates the creation of a pollen tube that grows inside the style, creating a path between the stigma and the flower's ovary. Once the tube is complete, the second cell divides into two sperm, which use the pollen tube to reach the ovule. One sperm joins with an egg to create a zygote, a fertilized egg; the second sperm fuses with other nuclei to form the endosperm, a food supply contained in the seed.
As time passes, the zygote becomes the embryo of the new plant. The seeds of flowering plants come in a great variety of sizes, shapes, and textures, but the embryonic plants contained within have the same basic design, with a rudimentary leaf or leaves, called cotyledons, as well as a root tip and a stem. (Cotyledons are the first leaves to appear after a seed sprouts; they are followed by the plant's true leaves.) Most flowering plants have two cotyledons, and thus are called dicotyledons, or dicots. The monocotyledons, or monocots, a smaller group, produce only one seed leaf when they sprout. Plants in the grass family, including corn and grains, are monocots.
In flowering plants, the seed is encased in the ovary, which enlarges into a fruit. The fruit may be red like an apple or a rose hip, green like a cucumber, purple like an eggplant, or brown like an acorn. In plants like tomatoes, the fruits become increasingly soft and fleshy as their seeds mature; in other plants, such as garden peas or winter aconite, the fruit wall becomes increasingly dry and eventually splits. Oriental poppies form hard, dry fruits that disperse seeds through small openings like a salt shaker's; other species, such as cherries, have a soft fruit that surrounds a hard pit, or stone, which contains a seed.
No matter what size or shape fruit it is encased in, once the seed, with its embryonic plant, has formed completely, growth stops and the seed typically enters a period of dormancy. The tiny plant consumes minute amounts of energy from food stored inside the seed—just enough to keep it alive, until it germinates and grows.
TOP: Seeds that stick to clothing or fur: needle grass, Aristida
oligantha (A); squirrel grass, Hordeum jubatum (B); with one seed at
right; and a pod of the unicorn plant, Proboscidea louisianica (C),
BOTTOM: Seeds dispersed by mechanical means: squirting cucumber, Ecballium elaterium (A); impatiens, Impatiens aurella (B); witchhazel, Hamamelis virginiana (C); and Cardamine hirsuta (D).
Spreading The Treasure
After the development of the seeds and fruit, the next order of business for any flowering plant is dispersal—getting the next generation out into the world where it can find suitable habitat to germinate and grow. Anyone who has ever tramped through the woods with a longhaired dog or wearing wool pants knows how some seeds travel: They become affixed to clothing and fur by using hooks, barbs, and even Velcro-like natural structures to guarantee their dispersal around the country, if not the world. The seeds of mistletoe are covered with a sticky substance and become attached to birds and other animals. We humans unintentionally spread seeds in other ways: as discarded foodstuff or as unintentional traveling companions, hidden in pants cuffs, or stuck to the mud on dirty boots and shoes—and we spread them intentionally by buying or collecting seeds and planting them in our gardens.
Plants don't require humans to spread their seeds, however. Small seeds often use the wind to blow them from place to place. Some seeds, like those of orchids, are dustlike; others have plumes, like dandelion or milkweed; or, as do the seeds of maple, they have wings that enable them to glide through the air.
Whether by rain, streams, or ocean currents, many seeds are distributed by water. Very small seeds, especially if they are light in proportion to their size, float. Corky seeds, like those of the carrot family, can stay afloat for weeks.
Seeds are also spread by animals that eat fleshy fruits. The seeds in these fruits often pass through animals' alimentary tracts unharmed; in some cases, the journey through the digestive system even speeds up germination. Beetles, ants, and scores of other insects carry seeds from one location to another. Finally, plants themselves are responsible for spreading seeds: A number of species, such as the yellow-flowered creeping oxalis, shoot their seeds into the air. The record is probably held by the West Indian swordbean, whose large pods snap open with a crack and propel their seeds at least 20 feet. Less impressive is the squirting cucumber, an annual vine whose fruits push their seeds out in a lump of semi-liquid mucilage. Not neat but effective.
Wherever they land, seeds can remain in suspended animation until conditions are right to spur germination. While some seeds, given water, will germinate immediately, many others follow an internal clock which insures, as much as possible, that when the seed does germinate, conditions will be conducive to its growth. For example, some seeds require a period of exposure to cold temperatures to break dormancy -- nature's way of making sure that seeds of plants in the temperate zone don't germinate until the killing cold of winter has passed. Gardeners need to simulate these natural conditions in order to get such seeds to grow. (See "Special Handling")
The seeds of some plants can remain dormant for months or even years. While the seeds of most species remain viable for only a few years (some for only a few days), others have remarkable longevity. Although many of the tales of sprouting seeds that were found in centuries-old tombs are apocryphal, there is good evidence that some seeds have astonishing durability. The record for longevity probably goes to an Arctic lupine from the Canadian Yukon—seeds germinated after they had been frozen and buried in an ancient rodent burrow for 10,000 years.
Scientists believe that the seed coat is the mechanism that allows a seed to be viable for so long. Typically, this structure consists of an outer and inner cuticle, often impregnated with waxes or fats, which are surrounded by one or more layers of thick-walled, protective cells that are so hard that it is difficult for water to seep into the interior and trigger germination. Sometimes this makes the gardener's job more complicated, requiring special measures to prompt a seed to germinate (some seeds won't germinate until the protective seed coat has been roughed up a bit, as it is when seeds pass through an animal's gut). But mostly, it's a boon, because seeds often must be stored for weeks or months, and in some cases years, before they can be planted.