130 



Bydration and Growth. 

 TABLE 102. Swelling of joints formed in 1916. 



No record was made of the temperatures of the swelling sections, 

 but these in general were determined by the seasonal conditions. 

 Thus, sections were swelled in November at 16 to 18 C. and at 25 

 to 28 C. in April and May. 



The measurements given in table 102 were made primarily for the 

 purpose of following the changes in the flattened stems of Opuntia as 

 they go toward maturity. The joints, having reached the mature con- 

 dition in November, the changes for the next 4 or 5 months are not 

 directly connected with growth, although the action of living cells is 

 concerned. The period of late summer is one of progressive desic- 

 cation, in which the water-loss is much greater than the supply 

 obtained from the soil, and this drying out continues until some time 

 in January and February, when the winter rains saturate the soil. 

 It would be expected that if sections of the partially desiccated 

 plants were taken during the dry period, the swellings which they 

 would show when placed in solutions would be a direct function of 

 the net water-loss which they had sustained, rather than of changes 

 in composition. It was therefore necessary to devise a method by 

 which the material could be reduced to a standard in which the 

 seasonal water-condition would be eliminated. This was accom- 

 plished by the use of dried slices from the median portion of the 

 joints in such manner as to exclude nodes and spines. 



The joints were laid flat on a table and against a strip of wood 5 

 or 6 mm. in thickness, which served as a guide for sliding a knife to 

 cut away one epidermal region. This being done, the piece was turned 

 over and a horizontal movement of the knife along the lowered guide 

 would cut away the other epidermal region, leaving a slice 3 to 5 mm. 

 in thickness, which was dried in the air at temperatures of 15 to 18 C., 

 using filter-paper or blotting-paper to keep the sections plane during a 

 part of the process. Sections cut from the dried pieces were calibrated 

 by scales of the type used commercially for measuring the thickness 

 of paper (fig. 8), and then swelled under the auxograph in the usual 

 manner. The calculation of the percentage of swelling on the final 

 thickness of the dried material eliminated the factors attendant upon 



