For many kinds of seeds, the rate of germination the rate of growth of the seedling increases with a rise of temperature until near the upper temperature limit for growth, when the rate of germination slows down.

In the early records, the temperature for the maximum rate of germination often was taken as the optimum temperature. Many individual seeds, however, may not germinate at all at the temperature of most rapid germination of other seeds.

One should consider therefore a compromise between highest percentage of germination and fastest rate of germination.

Most seeds germinate slowly at low temperatures. The reported minimum temperatures for germination therefore often depend on the patience of the observer. Several observations of seeds germinating on cakes of ice have been recorded.

Scientists now give emphasis to understanding the ways in which temperatures affect the germinating seeds rather than to establishing rigid limits of the overall effect of temperature.

The seeds of many plants will not start germination at high temperatures even though the seedlings will grow normally at high temperatures. In them, some step leading to the start of germination is blocked at higher temperatures but can proceed at lower temperatures. The temperature that keeps them from germinating varies with the kind of seed and the conditions under which the seeds mature.

Generally, the critical temperature is lowest just after harvest and gradually gets higher until the special temperature requirement disappears after a variable period of storage. Storing the seeds under very dry conditions and at very low temperatures delays such a change. Wheat often will not germinate at temperatures above 59 F. for 1 or 2 months after harvest. This condition varies with variety and with the weather at the time of maturity.

Most samples of lettuce seed will not germinate in soil that stays at high temperatures (76 to 86 ). The critical temperature becomes higher as the seed ages in storage, but very few samples of lettuce seed will germinate at 86 even though seedlings will develop at that temperature.

Seeds of lettuce and many other plants that are held in darkness for several days at a temperature too high for germination become especially dormant and then will not grow when transferred to a lower temperature that previously would have favored germination.

The seeds of some plants have an opposite response to temperature. Seeds of alyceclover (Alysicarpus vaginalis), a semitropical plant, will germinate only at a temperature of 85 or higher when planted soon after harvest, but a year later they will germinate readily at lower temperatures.

The germination of seeds of coffee and the best growth of the young seedlings are restricted to a narrow temperature range (near 80 to 85 ). The coffee tree thrives best at somewhat lower temperatures. The nurseries for young coffee plants therefore are usually at a lower elevation than the plantings of bearing trees.

The seeds of many grasses and flowers and of some-vegetables germinate poorly at any temperature that is constantly maintained at a uniform level, but they germinate well if the temperature is alternated between a lower and a higher temperature.

Bluegrass seed will germinate well if the temperature is kept at about 60 for 16 hours and at about 75 for 8 hours each day or even at a daily alternation between 68 and 86 . These seeds usually germinate in nature in spring when the night and day temperatures vary sharply.

A satisfactory explanation of the physiological changes involved in this requirement for daily changes of temperature has not yet been given.

SEEDS need to absorb a fairly definite amount of water before germination will start. Different kinds of seeds vary in their response to surrounding moisture during germination. This probably is associated with the influence of the surrounding moisture on the aeration of the seeds.

Rice will germinate under water and with a low supply of oxygen.

The seeds of cattail (Typha latifolia) and probably those of other water plants, instead of being sensitive to a lack of oxygen, germinate only when the supply of oxygen is reduced.

Many other seeds, including clovers, will germinate under water. Others, such as cabbage, do not germinate if even a film of water surrounds the seed.

Spinach is especially sensitive to excess moisture; the spongy covering that surrounds the seed can become filled with water and so reduce aeration and prevent germination.

LIGHT does not influence the germination of many kinds of seeds, but the germination of others is controlled by the presence or absence of light.

When fully moist seeds of certain varieties of lettuce are held in total darkness at 68 , few of the seeds will germinate. If the seeds are exposed briefly to light, all the seeds are stimulated to germinate. The flash from a photographic flashlamp, in fact, is enough to cause them to germinate.

The promotion of seed germination is brought about by red light of a comparatively narrow range of wavelengths.

If moist lettuce seeds are promoted by exposure to red light and are then exposed to far-red light (just at the limit of visibility), the promoting effect of the red light is reversed, and the seeds will not germinate. This promotion and inhibition can be repeated many times, and whether germination occurs or not depends on the band of light that is given last.

THIS REVERSIBLE photoreaction that controls seed germination has been demonstrated in at least 20 kinds of seeds. Undoubtedly it occurs in many more.

It is interesting that the same mechanism that controls the germination of some kinds of seeds is also responsible for the photoperiodic control of flowering, for the control of the elongation of seedlings, for the coloring of seedlings and of certain fruits, and for the control of other phases of the development of plants.

BOTH RED AND FAR-RED light are present in daylight and in most artificial light, but the red light has the strongest effect on many light-sensitive seeds like lettuce. The germination of such seeds is stimulated by unfiltered light, but seeds of henbit (Lamium amplexicaule) are so sensitive to far-red light that their germination is prevented by long exposure to daylight or incandescent light.

Germination of the seeds of many plants, like those of lettuce, is promoted by a single brief exposure to red light, but the seeds of many grasses require repeated light exposures over a period of several days to stimulate the germination of all the seeds.