macdougal: mechanism and conditions of growth 15 



The measurements of mature joints which had been made by 

 rulers, calipers, horizontal microscopes, etc., showed that changes 

 in dimension of some amplitude due to the amount of the water 

 balance occurred in these plants. The known factors to which 

 such changes might be due were the water supply, transpiration, 

 temperature, variations in acidity, etc., all of which would affect 

 the growth of young joints. 



It was important therefore that the general nature of the 

 changes be made out and the parts played by the contributory 

 factors analyzed. In accordance with this requirement the mature 

 joint bearing the new bud (No. 5) was fixed firmly in plaster with 

 its root-bearing base immersed in a vessel of water on March 4, 



Fig. 2. Auxographic tracings of the variations in length of mature joint of Opuntia 

 Blakeana, No. 5, preceding the development of the new joint and during the earlier 

 part of its growth, X 8|. Elongation produced a downward movement of the pen. 

 The record is complete for 42 days, starting in a period of equalizing daily variations, 

 including a period of enlargement and ending in a stage of decreasing length. 



1 91 5. An auxanometer lever was put in contact with a space 

 between two areolae on the apical margin and the features of 

 changes in volume are well shown by tracings of the actual record 

 given in fig. 2, the actual elongation being magnified twenty 

 times and denoted by downward movement of the pen. The 

 old joint had suffered the usual winter depletion of water as it 

 stood in the open, and its transferral to the experimental setting 

 duplicated the conditions under which it w^ould have begun to 

 take up water more rapidly from the moist soil. The increase of 

 the water-balance began to be manifest on the eighth day after 

 the experiment was begun (see uppermost tracing in fig. 2) and 



