THE ROLE OF THE SEED-COATS IN GERMINATION OF PHOTOSENSITIVE LETTUCE SEEDS

In order to analyze the role of both the fruit-coat and the endosperm in the germination of Grand Rapids lettuce seed and in the elongation of the embryo, the seeds were subjected to a number of operations. The fruit-coat was found to make only a minor con tribution to the photosensitivity of the seed. The fact that 100% germination of these seeds can occur in complete darkness, when the restraining influence of the endosperm is removed by appropriate cuts, makes clear that it is the integrity of the endosperm layer that lecides the photosensitivity of the seed to red and far-red light. Thus it is deduced that the effect of light is exerted primarily on the mechanical properties of the endosperm layer. Neither red nor far-red light was found to affect the elongation of the radicle of de-coated seeds, whereas both powerfully affect the germination of intact seeds. It follows that radicle elongation is not itself the limiting factor in germination, although of course it follows immediately upon the germination process proper. In order to account for the ability of red light to initiate germination, it is proposed that the final step in the germination control process is the production of an enzyme whose action enables the tip of the radicle to penetrate through the coat. This makes it possible for the radicle to begin elongating. Although the detection of such an enzyme in the radicle was not successful, the cotyledons do produce both a pectinase and an enzyme hydrolyzing carboxymethylcellulose. Furthermore, seeds injected with a cellulolytic, hemicellulolytic or pectolytic enzyme germinated to nearly the maximum extent in the dark. This gives considerable support to the hypothesis.