Dormancy and Germination of Seeds 265 



maple is another seed of this kind. When its water content falls 

 below 30 per cent, it dies. 



Dormancy is generally more pronounced in seeds produced in 

 late summer and autumn. Even when the seeds are kept in 

 conditions usually favorable for germination, they will not germi- 

 nate for several months — sometimes not for several years. 



External causes of dormancy. Dormancy may be due to 

 various causes. Some of these causes are environmental, others 

 He within the seed itself. In late autumn and winter the tem- 

 perature may be too low for germination. This condition often 

 prevails when the seeds of many late-flowering plants are mature. 

 On the other hand, the temperature may be too high, as it often 

 is when the seeds of cool temperature species of the desert mature. 



The seeds may fall on ground that is too dry, or they may fall 

 to the bottom of a pond where the oxygen content is very low or 

 insufflcient. They may be covered with earth, under conditions 

 that exclude oxygen entirely. Some seeds will not germinate in 

 the absence of Hght. Unfavorable temperature, too Kttle water, 

 the insufflciency of oxygen, are the most common external causes 

 that prevent germination in nature. 



Internal causes of dormancy. The failure to germinate, even 

 when external conditions are favorable, often depends on certain 

 characteristics of the seed coats or of the embryos. It is usually 

 possible to break dormancy when the cause is known, a fact that 

 is of great importance to horticulturists in their work of growing 

 seedlings. Many of these specific causes have been determined ; 

 some of the most important are given below. 



I. Seed coats impermeable to water. One of the commonest 

 causes of dormancy is the exclusion of water by hard seed coats. 

 The seeds of many water plants, and also of plants belonging to 

 the legume family (clover, alfalfa; and lupine), have coats of this 

 character. When placed in water no absorption takes place, and 

 germination is therefore impossible. In some species all the 

 seeds are hard and impervious to water ; in others, only some of 



