Even the isolation of an inhibitor is not direct evidence that it actually functions as an inhibitor in the seed. When a chemical is extracted, it is torn from its normal location and form in the cells.

Cyanide, a powerful (and deadly to animals) inhibitor, is a common constituent of many seeds. It is an obvious candidate for a compound that can control germination. In the embryo, however, it occurs in a chemical complex known as emulsin, a glucoside. In this form it has no growth-inhibiting properties. Free cyanide is formed only when the cells are damaged. Many investigations have failed to give adequate evidence that cyanide actually functions as a germination inhibitor.

There is a rigorous test that should be used to decide whether an extracted compound functioned in the seed as an inhibitor. The true inhibitor should change in quantity parallel to changes in the physiological condition of the seed. If the seed is strongly blocked, a germination inhibitor should be present in high concentration. As the block is removed, the concentration of inhibitor should decline to a minimum when germination is most prompt. If after-ripening is reversed by high temperature or lack of oxygen, the concentration of inhibitor should increase.

WITH THE present state of our knowledge, we can conclude that growth- inhibiting chemicals may very well control germination. They are not the only possible mechanism, however.

Much more intensive research will be required to establish their nature.

CHANGES in cellular organization exemplify a type of mechanism that has not yet been investigated enough as a possibility for controlling germination.

Modern biology recognizes that cells of animals and plants are similar in that they contain large numbers of complex chemical compounds. The compounds are not distributed at random within the cells.

They exist in definite chemical and physical combinations in parts of the cell, such as mitochondria and microsomes, which are too small to be seen with ordinary microscopes. Even powerful electron microscopes can only suggest how they are arranged.

We know that the processes of respiration, photosynthesis, and protein synthesis depend on such structures.

In the seed, is it possible that germination may fail because the compounds within these structures are incompletely or improperly joined? We have no direct information yet on this possibility, but we should not ignore it.

Many uncertainties and unknowns exist in our understanding of the mechanism of germination blocks.

What, in fact, do we know?

We know that we can distinguish between blocks that seem to yield easily to removal and those that require much more strenuous efforts. Light, for example, stimulates the germination of many seeds, and this same mechanism controls other plant growth responses, including flowering.

We know that the light reaction results from a pigment that may absorb either red or far-red light. Although the pigment has been isolated from seedlings, we do not know how it is coupled to germination.

We know that other factors also can stimulate germination. Virginia runner-type peanut seeds (Arachis hypogaea) are blocked at maturity. They germinate within 48 hours if exposed to air containing a minute amount of ethylene gas.

Some kinds of seed respond to environmental factors much more sluggishly. They do not seem to require a true stimulus. Time is important; the essential processes are slow. This is the case with low temperature.

We know that low temperature is required to remove the blocks in many seeds. This may require a few days to several months. High temperature can regenerate the block.

Since chemical reactions depend on temperature, increasing in rate with increasing temperature, we assume that temperature must influence one type of chemical reaction differently than does another. The specific reactions involved are completely obscure, however. Why low temperatures should be required is difficult for us to understand.

Interaction between blocks is common. Blocks that can be removed by stimuli (such as light) and blocks that can be removed by long exposure to low temperatures may be interdependent. Artificially applied chemicals can interact with light and temperature.

We know that these interactions exist. We know that germination is controlled by "blocks." It seems reasonable therefore to visualize germination proceeding by a number of alternate pathways. These pathways may be closed by blocks imposed and removed by various environmental conditions. Some of the pathways are temperature sensitive. Some are light sensitive. Some can be controlled by applied chemicals. Some cannot be.

The idea of alternate pathways is not unique.

Biochemistry has shown that chemical reactions in cells and organisms are brought about by enzymes. The rate of activity of these enzymes is controlled by many factors, including temperature. We do know that a cell may have several pathways of producing or utilizing a required chemical. These are alternate pathways of synthesis or destruction.

Are these also alternate pathways of germination? This question cannot now be answered, but we strongly suspect that the answer is yes.

Why bother with these complexities of mechanism? Are they important? Are they worth the time and energy spent in studying them? The answers to these questions must be yes.

Understanding in a scientific sense leads to control. Control of germination is of enormous practical importance. Think of agriculture without weeds!

We recognize that germination is a critical stage in the life of each plant. Natural mechanisms exist that control germination. An understanding of these mechanisms would provide us with the scientific basis upon which control of germination could be based.

With this knowledge we might cause, or prevent, germination at will. We also could probably control the storage life of seeds.

There is another reason for understanding the mechanism of germination. Scientists since 193o have come to recognize that all cells, whether they are bacterial, insect, plant, or human, are essentially the same. Blocks to growth are not unique to plants; they are known to exist in many, if not all, organisms.

An understanding of the mechanisms of germination blocks in seeds could contribute to a larger understanding and control of growth in the other organisms.

BRUCE M. POLLOCK is Leader of Vegetable Seed Investigations for the Crops Research Division of the Department of Agriculture. He has done post-doctorate research in the Cytochemical Department of the Carlsberg Laboratory in Copenhagen, Denmark, and was associate professor of biological sciences and horticulture at the University of Delaware.

VIVIAN KEARNS TOOLE is a plant physiologist doing research on seed physiology in the Vegetables and Ornamentals Research Branch, Crops Research Division, Agricultural Research Service. She holds degrees from the University of North Carolina and the George Washington University.