458 



PLANT PHYSIOLOGY 



(Fig. 142) and then are transferred to a place favorable for 

 growth. It is interesting to note that the effects of warm baths 

 and of ether are purely local. Accelerated development takes 

 place only in buds and branches that have been exposed directly 

 to the stimulants. When a lilac bush is placed in such a position 

 that only part of its branches are in warm water, the others 

 remaining outside, then in a few days a marked difference 

 between the development of these branches will be observed 



(Fig. 143). 

 Besides the treatments with ether and warm baths, several 



other methods used to induce the buds of plants to break their 



rest period have been recently 

 discovered. Plants may be 

 forced by various volatile sub- 

 stances, such as hydrocyanic 

 acid, acetone, ethylene, illumi- 

 nating gas, tobacco, and other 

 fumes. In Puerto Rico, it is 

 a common practice to smudge 

 fields of pineapples to force 

 them into bloom. Cutting of 

 the buds, injection of water or 

 salt solutions, exposure to 

 X-rays and radium emanation, 



Fig. i44.^N^ie kept d^ the and many other external influ- 

 winter (1) under snow, (2) in a warm enccs will hasten the breaking 

 greenhouse. ^^ ^^^^^ ^j^^^ ^ Considerable 



acceleration in development will be observed after freezing. 

 Plants that have remained during the winter in a warm room 

 will start to grow later than those subjected to the action of 

 frost (Fig. 144). 



Of the greatest practical importance are the methods of break- 

 ing the rest period of potato tubers. In many southern regions, 

 the vegetative period is long enough to obtain two successive 

 yields of potatoes during one summer, at least of the earUer 

 varieties, but this is seriously impeded by the fact that young 

 tubers remain dormant for several months after harvest. Most 

 fall-harvested potatoes are dormant until January or February. 

 By treating potato tubers with different chemical agents, thio- 

 urea, thiocyanates or ethylene chlorhydrin, Denny was abie to 

 produce rapid germination of freshly harvested potatoes. Of all 



