Delayed Germination in Seed of Alisma Plantago

Abstract

1. Dormancy in the akenes of Alisma Plantago is due to the mechanical restraint of the seed coat. This restraint enables the seed to be in water for years without germination. 2. The chaffy carpel wall plays no part in the dormancy. Of the three layers of the seed coat (the outer single layer of reddish-brown cells, the inner single layer of white cells, and the lining acellular pectic hemicellulose material), the outer seems to play no part in the delay. The effect must be attributed to one or both of the inner layers. 3. As the intact fruit lies in water in the saturated condition, the embryo itself does not half (probably not more than one-fourth or one-fifth) consummate its possible imbibitional and osmotic swelling. The embryo only partly swollen thus lies for years in water, restrained in its swelling by the seed coat, against which it must be exerting a pressure of approximately 100 atmospheres. 4. The air-dry seed (freed from the carpel wall) when placed in water swells rapidly. It increases 40 per cent of its air-dry weight in the course of two hours. From this time on it shows a slow increase to 50 per cent of its air-dry weight, which is maintained constant even after long periods of soaking. A large part of the water absorption is due to the hydrophilous pectic and hemicellulose substances of the seed coat, especially the inner acellular layer. 5. When the coat cap is removed from the large end of the embryo, thus leaving the embryo more free to continue its imbibitional and osmotic swelling, the seed swells even much more rapidly, reaching about 60 per cent of its air-dry weight in two hours and more than 100 per cent after 20 hours. The imbibitional and osmotic swelling gradually passes into growth enlargement. 6. With the coat cap removed from both ends of the embryo and the seed placed in water, the embryo elongates 19 per cent of its air-dry length in 2.5 hours. This would extend the embryo at least 20 per cent the length of the swollen seed beyond the limits of the seed coats. This elongation is all imbibitional and osmotic, involving no growth. Five hours' soaking gives an elongation of 30 per cent of the air-dry length of the embryo. This is only in very small part due to growth. Sixteen hours' soaking gives 36 per cent elongation, which involves considerable growth. Imbibitional and osmotic swelling alone would extend the embryo of Alisma far beyond the limits of the swollen seeds. 7. The seed coat is composed almost entirely of pectic substances which are very easily transformed by weak acids and base. Besides bringing about chemical changes in these substances, acids and bases change their water relations as is true of hydrophilous colloids in general. 8. There is some evidence that acid increases the imbibitional force of the embryo. If such is the case, the increase is very slight. Bases increase greatly the rate of elongation of embryo. 9. It seems that the effect of acids and bases on the germination of Alisma seeds is largely to be explained by a weakening of the seed coats, so that the imbibitional and osmotic swelling of the embryo is capable of breaking away the coat cap at the large end of the embryo. It is possible that they are also in part effective by increasing the force of the imbibitional and osmotic swelling. 10. This gives a chemical-physical explanation which displaces the vague implications of the term "stimulus." 11. The seeds of Alisma, as of the seeds of water plants in general, are capable of lying in water for years in the imbibed condition without losing their vitality. In contrast to this, seeds of land plants will withstand such storage for a relatively short time. 12. The embryo of Alisma, at the expense of its stored foods alone, is capable of more than 120 per cent elongation in total absence of oxygen. For various other phases of its development (greening, branching, development of primary root) it requires some free oxygen. The greening requires at least 5 mm. of air pressure, the branching more than 5 cm. of air pressure, and the general development of primary roots still more.