RESTING PERIODS 297 



its leaves yearly when transplanted to the tropics, or when it is 

 cultivated in a tropical glass house, as may be easily demonstrated 

 with Fagus, Alnus, or Acer. The maintenance of approximately 

 unchanging climatic conditions around the plant will cause some 

 species to lose the yearly rhythm ultimately, although most kinds 

 persist in all of its manifestations under all conditions. When 

 the suspended conditions are again allowed to act upon the plant 

 it resumes the rhythmic habits. 



The daily rhythm is also more or less deeply implanted in the 

 movements of protoplasm. Thus it has been seen that the nycti- 

 tropic movements persist in uniform darkness and temperature, 

 and that the repetition of the maximum of daily growth occurs 

 some time after the conditions which have been induced become 

 non-operative. 



374. Modification of the Grand Period of Growth. Attach a 

 leaf of Arisaema to an auxanometer, and record the course of the 

 grand period of growth, until the rate decreases to a minimum of 

 less than a millimeter daily. Now remove the whole preparation 

 to a dark room of the same temperature and adjust anew. Note 

 that a second period of growth ensues which follows about the 

 same law of acceleration and decrease as the original effort of 

 the leaf. This shows that growth, whether developmental, or in 

 response to certain stimuli, is rhythmical. 



375. Resting Periods. Species which have become habituated 

 to alternating periods of rest and activity acquire the power of 

 assuming a resting condition during a certain portion of the year, 

 and only a few species have been found which would relinquish 

 the resting period when removed to the tropics. In some the 

 alternation of cold and warm seasons, or wet and dry periods, has 

 become absolutely necessary, and they may not continue exist- 

 ence without such changes. In others, the absence of the con- 

 ditions attendant upon the resting period will result in enfeebled 

 development. Thus, for instance, the chemical and stimulating 

 action of low temperatures seems quite necessary to the devel- 

 opment of many seeds and bulbs. The duration of these resting 



