After-Ripening and Germination of Seeds of Tilia, Sambucus, and Rubus

Abstract

General.--Air-dry seeds of Tilia americana, Sambucus canadensis, and Rubus Idaeus do not germinate when placed on a moist substratum at room temperature. In no case does water absorption seem to be the limiting factor. Air-dry seeds planted in the soil over winter give low percentages of germination. Tilla--Seed coats are not the cause of dormancy, although they may serve to lengthen the dormant period. A state of dormancy exists in the endosperm or embryo, or both. Seeds with coats removed after-ripen at temperatures slightly above freezing. At 0-2⚬ C. seeds after-ripen, but do not germinate. At 4-6⚬ C. both after-ripening and germination take place. Seeds after-ripened at 0-2⚬ C. germinate readily at 10-12⚬ C., but very poorly at room temperature. Once germination has begun growth proceeds best at temperatures above 12⚬ C. As after-ripening, progresses the hydrogen ion concentration increases, as do also the water holding capacity and the oxidase and catalase activities. The greatest amount of free acid is present in the germinating seeds. Autodigestion of pulverized seeds shows the greatest acid increase in the after-ripened ungerminated seeds. This is probably due to their high lipase activity. Sambucus.--As high as 77 per cent of germination was obtained by layering fresh seeds out of doors over winter. No satisfactory forcing agent has yet been found. A slight forcing effect of several acids, bases, and salts has been observed. The best of these forcing agents are nitrates and sulphates. Although Sambucus seeds are probably injured by drying, that is not the only factor to be considered, since freshly gathered seeds with a moisture content of 22 per cent will not germinate when placed on a moist substratum. As yet it has been impossible to approximate perfect germination, and much still remains to be learned concerning the conditions necessary to reach it. Rubus.--Dormancy is probably due to the high breaking strength of the endocarp. Seeds treated with concentrated sulphuric acid for 2 hours, then thoroughly washed, germinate readily on cotton, filter paper, or quartz sand. The optimum temperature for germination lies between 20⚬ and 25⚬ C. Seeds germinate equally well in light or darkness. Naked seeds germinate poorly iff soil. This may be due to the action of fungi, bacteria, or to other causes as yet unknown. As a practical method for the germination of Rubus seeds, if one is not to resort to layering, the writer suggests the following: The seeds should be removed from the pulp as completely as possible. If the berries are crushed and then thrown into water most of the pulp can be floated off. The pulp still clinging to the seeds may be removed by allowing fermentation in water to take place or by treating the seeds with a 5 per cent solution of sodium hydroxide for 15-20 minutes, after which they should be thoroughly washed in running water. It is essential to dry the seeds for at least 24 hours, or the treatment with concentrated sulphuric acid which follows will result in heating. The seeds should be left in the acid for approximately 2 hours. In order to obtain uniform results it is advisable to use a large excess of acid and to prevent the seeds from gathering in clumps or layers. Frequent stirring is essential. By rubbing a few of the seeds in the palm of the hand from time to time it is possible to determine when the entire endocarp on a majority of the seeds has been carbonized. When this point is reached the acid should be drained away and the seeds thrown into an excess of cold water. It is advisable to change the water frequently or to put the seeds in running water, where they should be left for at least 15 minutes. When they are removed from the water they should be treated with an excess of a 5 per cent solution of sodium bicarbonate until bubbles cease to rise, after which they may be washed in running water for 15 minutes. In order to remove the carbonized endocarp the seeds may be placed on filter paper and rubbed under the fingers. It is impossible to remove the endocarp if it has been allowed to become dry following the last washing. The writer is indebted to Dr. William Crocker and Dr. Sophia H. Eckerson for many helpful criticisms and suggestions during the progress of the work.