ADJUSTMENT TO TEMPERATURE 111 



tuates greatly at different times, or ceases at the end of a certain 

 period. The division of the year into a growing period and a 

 resting period is the most striking evidence of the law of growth. 

 Variations in growth probably arose originally from the fluctua- 

 tion of the controlling factors, heat and water. They have become 

 so firmly impressed upon the plants through the action of simi- 

 lar fluctuations upon countless generations that they are more 

 or less firmly fixed as habits which persist under changed condi- 

 tions. In consequence, rhythm appears to depend as . much or 

 even more upon the habitual development of the plant as upon 

 the physical factors of its habitat. 



The daily growth of a plant shows a maximum and a mini- 

 mum, the latter sometimes falling to zero. The maximum usually 

 occurs late at night, apparently after translocation becomes less 

 active, and the minimum falls near noon, not far from the period 

 of greatest photosynthetic activity. Between the two the rate 

 of growth falls and rises more or less gradually, though abrupt 

 changes often occur. During the growing period growth begins 

 slowly, but after a certain period, the length of which varies for 

 different plants, it rises rapidly and uniformly, as a rule, to the 

 maximum. The latter rate is maintained for a short time only, 

 after which it first falls rapidly and irregularly to a point near 

 the minimum. Growth is then maintained at a very low, irregu- 

 lar rate for some time and finally stops altogether. Cessation of 

 growth may occur but once in the lifetime of an individual, as 

 in the case of annual plants and most thallophytes, or it may 

 occur at fixed intervals, as in perennial herbs and woody }jlants. 

 In the latter the rhythm shown in a single period of growth and 

 of rest lasts throughout the entire year, although unusual condi- 

 tions may cause an interruption of growth at any time. Changes 

 or conditions of the plant which are connected with the resting 

 period become fixed habits, owing to their constant recurrence. 

 This is equally true of the adult jilant and of the embryo in the 

 seed. Woody plants which shed their leaves at the beginning 

 of an annual period of cold or drouth often retain this habit after 

 the cause is removed. Seeds which have been regularly exposed 

 to winter conditions have acquired the habit of remaining dor- 

 mant during this period. When brought under the usual condi- 

 tions they may refuse for a long time to germinate unless thoy 

 are subjected to cold naturally or artificially. The daily rh\-thm 



