18 Richard H. Boerker 
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 
