Imbibition and Growth of Opuntia. 



129 



across the epidermal surface and from 6 to 11 mm. in thickness were 

 arranged in a triangle in the bottom of a Stender dish and a triangle of 

 thin sheet-glass arranged to rest its apices on the three disks (see fig. 1) 

 The vertical swinging arm of an auxograph was now adjusted to a 

 shallow socket in the center of the glass- triangle, while the pen was set 

 at zero on the recording sheet. Water or a solution being poured into 

 the dish, the bulb of a thermometer was adjusted in it and the course 

 of the swelling was traced, the record showing the average result of 

 the action of the trio of specimens. 1 



FIG. 23. Calipers used in obtaining thickness of trio of sections. 

 Swelling is calculated on average. 



Extensive records of the growth of the flattened joints of certain 

 platyopuntias having been made at the Desert Laboratory, and as a 

 series of analyses of the carbohydrate constituents of these plants was 

 being carried out, parallel measurements were planned which might 

 show possible connection between the composition of these members 

 and their imbibition or swelling reactions. The first series began with 

 the young joints which are formed in April and May, reaching maturity 

 in about 40 or 50 days and extending through the seasons, including 

 the dry foresummer, the summer rainy season, then the dry after- 

 summer, merging into the winter with its final rainy season. The im- 

 bibition capacity of sections taken from the joints ran a course shown 

 in table 102. 



1 MacDougal, D. T. Imbibitional swelling of plants and colloidal mixtures. Science, 44: 502- 

 505. 1916. Also MacDougal. Mechanism and conditions of growth. Mem. N. Y. Bot. Gar- 

 den, 6: 5-26. 1916. 



