176 S. KLEIN 



Frequently the presence of these factors is not sufficient, and 

 other conditions have to be fulfilled. Given the necessary 

 environmental conditions, growth will be resumed after a 

 certain time, and the radicle, or in some cases the hypocotyl, 

 will start to penetrate through the coverings of the dispersal 

 unit; from then on processes of growth and development will 

 usually continue. 



We define as germination processes those which occur from 

 the moment the factors necessary to break dormancy are 

 supplied until the time when growth is resumed (considerable 

 time may elapse before the radicle protrudes through the seed- 

 coat-""*). This allows us to differentiate between processes 

 occurring during resumption of growth and continuation of 

 growth, a distinction the usefulness of which has been pointed 

 out repeatedly^. Unfortunately, since the most spectacular point 

 in all of these processes is the protrusion of the radicle, there has 

 been a tendency to regard this as the starting point of germination. 

 Many papers dealing with 'changes occurring during dormancy- 

 breaking' actually describe changes occurring during growth. 



The study of seeds which germinate only after some special 

 condition is fulfilled throws light on particular steps of the 

 germination process^. Let us, therefore, examine some of the 

 different factors which may be necessary for the germination of 

 seeds (for a more complete coverage of this topic, see^' ^' ^). 



Removal of, or treatment of the surrounding layers will in 

 some cases be effective. The reason for the incapacity of the 

 embryo to develop, even when the three 'primary' requirements 

 — water, oxygen, optimal temperature — are present, may be 

 located not in the embryo itself, but in the sun'ounding coats, 

 and by removing them completely or partially, or by otherwise 

 treating them, the germination block may be overcome. The 

 ways in which the surrounding layers may influence the embryo 

 are various : the layers may be hard and impermeable to water, 

 as in the seeds of the Leguminosae, thereby pi eventing germi- 

 nation. Or they may decrease gaseous exchanges as e.g. in the 

 cocklebur and many grasses. 



