Seeds

A seed contains all of the genetic information needed to develop into an entire plant. It is made up of three parts (Figure 22):

• The embryo is a miniature plant in an arrested state of development. It will begin to grow when conditions are favorable.
• The endosperm (and in some species the cotyledons) is a built-in food supply (although orchids are an exception), which can be made up of proteins, carbohydrates, or fats.
• The seed coat, a hard outer covering, protects the seed from disease and insects. It also prevents water from entering the seed and initiating germination before the proper time.

Germination

Germination is a complex process whereby a seed embryo goes from a dormant state to an active, growing state (Figure 23). Before any visual signs of germination appear, the seed must absorb water through its seed coat. It also must have enough oxygen and a favorable temperature. Some species, such as celery, also require light. Others require darkness.

If these requirements are met, the radicle is the first part of the seedling to emerge from the seed. It develops into the primary root and grows downward in response to gravity. From this primary root, root hairs and lateral roots develop. Between the radicle and the first leaflike structure is the hypocotyl, which grows upward in response to light.

The seed leaves, or cotyledons, encase the embryo. They usually are shaped differently than the leaves the mature plant will produce. Monocots produce one cotyledon, while dicots produce two.

Because seeds are reproductive structures and thus important to a species' survival, plants have evolved many mechanisms to ensure their survival. One such mechanism is seed dormancy. Dormancy comes in two forms: seed coat dormancy and embryo dormancy.

In seed coat dormancy, a hard seed coat does not allow water to penetrate. Redbud, locust, and many other ornamental trees and shrubs exhibit this type of dormancy.

A process called scarification is used to break or soften the seed coat. In nature, scarification is accomplished by means such as the heat of a forest fire, digestion of the seed by a bird or mammal, or partial breakdown of the seed coat by fungi or insects. It can be done mechanically by nicking the seed coat with a file, or chemically by softening the seed coat with sulfuric acid. In either instance, it is important to not damage the embryo.

Embryo dormancy is common in ornamental plants, including elm and witch hazel. These seeds must go through a chilling period before germinating. To break this type of dormancy, stratification is used. This process involves storing seeds in a moist medium (potting soil or paper towels) at temperatures between 32° and 50°F. The length of time required varies by species.

Even when environmental requirements for seed germination are met and dormancy is broken, other factors also affect germination:

• The seed's age greatly affects its viability (ability to germinate). Older seed generally is less viable than young seed, and if it does germinate, the seedlings are less vigorous and grow more slowly.
• The seedbed must be properly prepared and made up of loose, fine-textured soil.
• Seeds must be planted at the proper depth. If they are too shallow, they may wash away with rain or watering; if too deep, they won't be able to push through the soil.
• Seeds must have a continual supply of moisture; however, if over-watered, they will rot.

Many weed seeds are able to germinate quickly and under less than optimal conditions. This is one reason they make such formidable opponents in the garden.