1 8 Richard H. Bocrker 



which insulates the embryo from heat, water and air and protects 

 it from mechanical injury. Cottonwoods, willows, elms, soft 

 maples, and white oaks have a very short period of rest. Usually 

 the period is not over six months, but basswood and hornbeam 

 lay over from fifteen to eighteen months. It has likewise been 

 noted that some tree seeds must lay over for a certain period 

 before germination can take place. The common experience of 

 attempting to germinate seeds in mid-winter which have been 

 gathered during the previous fall is proof of this phenomenon. 

 This leads me to a brief discussion of the process of after-ripening 

 as it is called. 



Many seeds we know require a long "time for germination in 

 spite of the fact that they are surrounded by the proper condi- 

 tions. During this period it has been found that certain chemical 

 and physical changes take place which are necessary before the 

 seed can germinate. The length of delay is apparently de- 

 termined by the persistence of the structure of the seed-coat 

 and to the conditions under which the seed is exposed. The 

 term " after-ripening " has come into use to designate the changes 

 in the seed during this period. Eckerson (17) concludes that 

 most cases of delayed germination are due to the exclusion of 

 water or oxygen by the seed coats. But some seeds do not 

 germinate after all coats have been removed and the seed put into 

 germinating conditions, indicating that the delay is due to embryo 

 conditions. , It is now certain that some changes within the 

 embryo are necessary for germination. In the case of Crataegus 

 used by Eckerson it was found that food is stored in the embryo 

 in the form of fatty oils ; neither starch nor sugar is present. 

 A series of metabolic processes takes place in the embryo during 

 the period of after-ripening. At first there is increased -acidity 

 accompanied by increased waterholding capacity. There follows 

 an increased activity and production of enzymes and as a result 

 the fats decrease and sugars appear. The appearance of sugars 

 which are soluble and diffusible marks the beginning of the germi- 

 nation of the seed. 



All recent investigations both in America and abroad show how 

 extremely complex is the role of oxygen in germination. A set 



