LIGHT and temperature and other factors that influence germination are interdependent.

Seeds of some samples of tobacco will germinate very little at temperatures held constant at any temperature from 59 to 86 unless the moist seeds have been exposed briefly to light. At daily alternations of temperature between 59 and 77 or between 68 and 86 , however, germination is complete either in light or in darkness.

The seeds of peppergrass (Lepidium virginicum) germinate only after the moist seeds have been exposed to light, but even after full promotion by red-light only a part of the seeds germinate at constant temperatures, such as 59 or 68 .

If seeds of peppergrass that have been allowed to absorb moisture in darkness at about 70 for 24 hours are exposed to light and then returned to 70 in darkness, only about one-third of the seeds germinate. If, however, at the time the seeds are exposed to light, the temperature is raised to 95 for 2 hours, all of the seeds will respond to the light stimulus and germinate at 70 .

The short period at the high temperature removes some block that prevented the seeds from responding to the light stimulus. This short-time, high-temperature treatment is effective with a number of kinds of seeds in increasing the response to treatment with light.

When lettuce seeds of varieties that ordinarily germinate completely in darkness are held fully moist at a high temperature (86 to 95 ) for 1 to 2 days in darkness, they will not germinate if placed at a low temperature (59 to 65 ) that originally would have been very favorable for germination.

These dormant seeds, however, will germinate if they are exposed briefly to red light and are placed at the low temperature. The prolonged treatment at the high temperature changed the seeds from light insensitive to light requiring These examples of the interaction of the different germination requirements indicate that one should not consider any of these requirements separately. They all must be considered together to understand how to obtain the best germination of seeds.

THE Various requirements of different kinds of seeds have been learned from laboratory experiments, but this knowledge helps us to understand the behavior of seeds in the garden and in nature.

It is customary to give little or no soil covering when sowing many kinds of small seeds. Often it is stated that the seeds are planted near the surface because the seedlings are not strong enough to come up through much soil. The real explanation for most seeds is that they need to be exposed to light in order to germinate.

Weeds often appear in meadows where they had not grown for many years. They always appear where soil is brought to the surface by wheel tracks or other disturbance of the surface.

A simple experiment illustrates what takes place.

Seeds of peppergrass are planted on the surface of moist soil in three flowerpots. The seeds in two of the pots are quickly covered with about one-fourth inch of moist soil. The seeds in the other pot are left uncovered. Drying out is prevented by covering all the pots with a pane of glass. Abundant peppergrass seedlings appear in a few days from the seeds that were not covered. No seedlings appear in the other pots. If one then draws a pencil through the soil covering the seeds in one pot, seedlings appear in the disturbed area after a few days.

THE SEEDLING after germination must establish itself in the soil. The young plant is not fully independent. It is dependent on the reserve foods of the seed for further development of root, stem, and leaves until it can become established and can manufacture enough food for all its needs.

This means that the enzyme system of the germinating seed must continue to digest the starch and oil and protein of the cotyledons or endosperm. These digested materials must be moved to the growing regions. Here other parts of the enzyme system must release and make energy available.

The temperature, moisture, and other requirements for the establishment of the seedlings in general are the same as for the future development of that particular kind of plant. The structure of the seedling must be complete, however, to allow for the development of a normal, useful plant.

If some part of the seed has been injured by handling or by poor conditions of storage, the seedlings may be incapable of developing into a useful plant it is an abnormal seedling.

Equal vigor is not manifested in all seedlings, even the ones that will produce normal plants. When seeds are stored under unfavorable conditions, the first evidence of deterioration is slower germination and slower growth of the seedling. Seeds that have been harvested before fully mature may germinate, but the seedlings often lack normal vigor.

A person with experience in the germination of seeds can detect many of the seed lots that will grow into seedlings that lack normal vigor. A dependable method of determining the relative vigor of seed lots has not been developed.

OUR present knowledge of the details of the changes that take place in the developing seed and during germination is not sufficient for us to know what determines seedling vigor.

There is much interest at present in learning how to insure that seeds that are produced will be of good vigor and how to detect, in the seedling stage, which seedlings will produce the most vigorous plants.

EBEN H. TOOLE is a collaborator with the Crops Research Division and part-time consultant with the Asgrow Seed Co. of New Haven, Conn. Dr. Toole formerly was principal physiologist in charge of Vegetable Seed Investigations of the Vegetables and Ornamentals Research Branch, Agricultural Research Service. He was engaged in research on seed physiology from 1920 to 1959 in the Department of Agriculture.

VIVIAN KEARNS TOOLE, a plant physiologist, does research on seed physiology in the Vegetables and Ornamentals Research Branch, Crops Research Division, Agricultural Research Service. She has conducted research on seeds in the Department of Agriculture since 1930.