SEASONAL PHENOMENA IN THE LIFE OF PLANTS 459 



these substances, ethylene chlorhydrin is the most practical, 

 owing to its cheapness and convenience in manipulation. The 

 tubers are soaked for several minutes in dilute solutions. In 

 Russia, experiments with ethylene chlorhydrin have been suc- 

 cessfully carried out at several experiment stations. Treatment 

 with ethylene chlorhydrin not only breaks the rest period of 

 potato tubers and accelerates their germination, but it likewise 

 contributes to earlier tuber formation in plants that have devel- 

 oped from treated tubers. Northern-grown seed potatoes are 

 used in the coastal Gulf States for planting after their rest 

 period has been broken. This may be done by sprinkling 

 ethylene chlorhydrin in the railway cars after loading. 



The mechanism of the effects of these stimulants is unknown. 

 Some authors are of the opinion that they increase the permea- 

 bility of protoplasm and thereby induce growth. Others hold 

 that they increase the oxidation processes in the cell and thus 

 stimulate it« vital activities. By studying the effects of ether 

 treatment at different times in the fall and winter, Johannsen 

 came to the conclusion that its influence is not the same at differ- 

 ent periods. This he considers an indication that the state of 

 rest likewise varies. He distinguishes three conditions of rest : a 

 preliminary one, a stage of deep rest, and a condition of enforced 

 rest, which gradually and imperceptibly merge one into another. 

 During the preliminary stage, through which most buds pass in 

 summer, they can be readily returned to active growth. This 

 may be accomplished simply by removing the leaves. In some 

 trees, such as oaks and elms, the buds frequently open in summer, 

 even without special treatment, and produce secondary growth. 

 In the fall, after the leaves have normally dropped, bud dormancy 

 is in its deepest stage. At this time, forcing requires particularly 

 strong stimulants. Toward late winter and early spring, buds 

 gradually leave the state of rest. When placed under conditions 

 favorable for growth, they will begin to unfold without any 

 particular treatment. The stage of enforced rest and the absence 

 of growth at this period are no longer determined by internal 

 conditions but solely by unfavorable environmental conditions. 

 During this period, the usual stimulants will not hasten the 

 unfolding of buds. 



105. Conditions of Seed Gennination. — The mature seed 

 generally has a period of rest. Germination necessitates a 

 change to active growth. As in buds, this resting condition 



