44 GROWTH OF PLANTS 



tion medium just above freezing in order to after-ripen the seeds prepara- 

 tory to germination. Some seeds require low temperatures for germination, 

 while others germinate only at high temperatures, and finally others germi- 

 nate well if the temperature fluctuates rather ^^^dely from day to night. 

 There are other factors active in the soil that might either stimulate the 

 germination of dormant seeds or prolong their dormancy. Nitrates and 

 other nitrogen compounds generally present in the soil promote the germi- 

 nation of certain seeds. This is especially true of many of the light- 

 stimulated seeds.^^ Deep burial in the soil excludes light. Light-stimulated 

 seeds, such as celery, tobacco, and timothy, have been mentioned above 

 as remaining in the soil for several years in the dormant condition. 



Why do seeds of so many wild plants lie in a germinator or in the soil 

 in conditions that are favorable for germination of seeds of our cultural 

 plants and fail to germinate even after years? The answer to this question 

 is very complex and involved. The next chapter, which deals in large part 

 with types of dormancy in seeds, will throw much light upon it. At this 

 point we shall discuss only two factors that may play a part in keeping 

 seeds of wild plants dormant in the soil: (1) inhibiting chemicals in the 

 fruits, seed coats, or seeds themselves, and (2) unfavorable germination 

 conditions in the soil that throw seeds into secondary dormancy. 



Many investigations ^^' ^^' ^^' ^^- ^^- ^^' ^^' ^^ show that fleshy and dry 

 fruits, seed coats, and even the living parts of seeds contain chemicals 

 which inhibit germination of seeds. For instance, the fleshy fruits of the 

 tomato, cucumber, pawpaw {Carica papaya), and many other plants con- 

 tain chemicals that inhibit the germination of the seeds while they are 

 within the fruits, and this mthout any permanent injury to the seeds. 

 The woody material of the seed balls of beets and the coats of lettuce seeds 

 contain chemicals that inhibit germination. This becomes very noticeable 

 if batches of either are germinated repeatedly on the same moist filter 

 papers so that the inhibiting chemicals become concentrated. The em- 

 bryos and endosperms of some seeds also contain inhibiting chemicals. 

 Mostly these chemicals are not specific in their action but inhibit other 

 species of seeds as well as the species in which the chemicals develop. In 

 some cases the chemicals are more effective as inhibitors on seeds of other 

 species than on those of the species producing the chemicals. Some of 

 these chemicals are volatile and others are not. They are mostly heat- 

 stable, although some workers claimed to find inhibitors that are destroyed 

 at the boihng point of water. Some of the substances that have been 

 found in fruits and seeds that act as germination inhibitors are ammonia, 

 hydrocyanic acid (from amygdalin of rosaceous seeds), essential oils, alka- 

 loids, glycosides, and an unidentified substance, "Blastokolin." "^ These 

 inhibitors are more effective in ordinary germinators than in the soil, for 

 the soil moisture allows them to diffuse away from the seeds; also soil, like 

 animal charcoal, adsorbs the inhibitors and removes them from action on 

 the seeds. 



