﻿2 66 



BOTANICAL GAZETTE 



Historical 



As examples of the extreme views in the older literature, we may 

 mention those of Grisebach and Askenasy. The former con- 

 sidered the yearly periodicity of plants of the temperate regions 

 entirely a hereditary property, induced probably by physiological 

 selection due to alternating external conditions through a long series 

 of years; whereas the latter asserted that it is due directly to exter- 

 nal conditions. Sachs believed that the rest period is caused by a 

 deficiency of soluble food, and the cessation of the rest is due to a 

 gradual production of enzymes. He does not state whether internal 

 or external causes are responsible for the deficiency of food, nor does 

 he suggest the nature of the process which increases the enzymes. 

 Fisher (8) studied the carbohydrate transformations in woody 

 stems during the rest period, and concluded that periodicity of 

 growth is conditioned by a periodicity of the processes of food 

 changes, which in turn rest upon a hereditary periodicity of certain 

 properties of the protoplasm. 



Coming now to the more modern literature, the studies of 

 Muller-Thurgau and Schneider-Orelli (19-22) are of im- 

 portance in the solution of this problem. They think the winter 

 rest of plants is not conditioned by low temperature only, but the 

 cessation of growth rests also upon the internal properties of the 

 protoplasm of the meristem. These internal properties are in- 

 herited effects of winter cold on the growth process which after a 

 long time become fixed in the protoplasm. 



Howard (10) experimented with a large number of trees and 

 shrubs, all deciduous species native to the temperate zone. He 

 first tested their ability to grow in winter under favorable green- 

 house conditions and then subjected to artificial treatments those 

 which failed. Discussing the results of the foregoing experiments, 

 the author states: "This shows conclusively that the majority of 

 species, indigenous to the temperate climates, do not have firmly 

 fixed winter rest periods from which they cannot be awakened." 

 Although his observations were made on plants in winter rest, he 

 includes also summer rest periods in his general conclusions. He 

 thinks both forms of rest are caused by unfavorable external con- 

 ditions. If these conditions, such as cold or drought, occur at regu- 



