266 General Botany 



them. In either case the germination of a given crop of seeds, 

 under natural conditions, would take place over a series of years. 

 Red clover seeds, for example, when placed in water do not all 

 swell at once, but only a few at a time ; some may remain hard, 

 yet alive, at the end of ten years. It has been found that all these 

 seeds will germinate at once if the seed coats are scratched, broken, 

 or removed by being shaken together with sharp sand. Ninety 

 per cent of these clover seeds may be germinated immediately 

 also by immersing them in boiling water for 50 to 60 seconds, or 

 by pouring strong sulfuric acid over them and then after 30 to 

 50 minutes carefully washing them with a 5 per cent solution of 

 sodium bicarbonate (baking soda) and water. In this kind of 

 dormancy the embryo is in a condition to grow when mature, 

 but it cannot do so until the seed coat allows water to enter. In 

 nature, freezing and thawing, bacteria, and fungi are important 

 agencies in breaking the coats of this class of seeds. 



2. Seed coats mechanically resistant. In many seeds the seed 

 coat is a membrane strong enough to prevent the expansion of 

 the embryo by the absorption of water. Although such seed 

 coats are permeable, when put in water only a small amount 

 enters. The seeds of the common pigweed, water plantain, 

 black mustard, shepherd's-purse, and peppergrass furnish ex- 

 amples of this kind of dormancy. In nature the dormancy of 

 such seeds is broken by chemical changes which weaken the 

 seed coats as they lie on the soil. Dry storage promotes changes 

 of the same kind, but if the seeds are stored in water or wet soil 

 they remain dormant for many years. This explains why they 

 are such persistent weeds in gardens, even in gardens that are 

 kept clean for a number of years. So long as the seeds lie deep 

 in the soil they will not germinate ; but when brought to the sur- 

 face by cultivation, the coat dries out, and as soon as they are 

 again moistened germination follows. 



3. Seed coats impermeable to oxygen. In a third class of seeds 

 dormancy is brought about by the exclusion of oxygen. The 



