MACDOUGAL AND SPOEHR— GROWTH AND IMBIBITION. 309 



in this work present an identical type of respiration. That the 

 course of growth was similar in its general features was evidenced 

 by the records of the two plants which were under observation for 

 some time. Elongation begins with full daylight and assumes its 

 highest rate near midday and then checks abruptly about i P.M. 

 Shrinkage continues from this time until daylight of the following 

 morning. The end of the growing period is marked by a decrease 

 of the midday elongation and by increased shrinkage which equal- 



FiG. 3. Auxographic records of growth of joint of Opuntia versicolor. 



A. Record April 10 to April 15, 1916, rapid midday elongation of joint 

 near the maximum of its grand period. The first occurrence of shorten- 

 ing at 5". 



B. Record from April 18 to April 22, 1916. Slight diminution of daily- 

 growth and accentuated contraction at night. The temperature record applies 

 to this period. 



C. Record from April 25 to April 29, 1916. Increasing reversible varia- 

 tion in length with cessation of growth. 



ize each other while allowing a great total variation in length (see 



Fig- 3)- 



The general facts as to alterations in volume of Opuntia by 

 growth and other changes, including shrinkage, are in accordance 

 with those previously described.'^ Elongation takes place chiefly in 

 the first half of the day both in mature and growing joints. Shrink- 

 age, slackening or stoppage of growth ensues after midday and 

 continues for a varying period which may extend until the follow- 

 ing morning. The type of respiration of these plants is one in 



*5 MacDougal, D. T., " Mechanism and Conditions of Growth," Mem. N. 

 Y. Bot. Garden, 6: 5-26, 1916. 



