Imbibition and Growth of Opuntia. 



139 



tors is to be seen late in the afternoon, with lessening water capacity, 

 minimum acidity, and the beginning of decreased absorption by the 

 root-system. 



Some measurements of growth and hydration of the Platyopuntia, 

 the growth of which has been observed so extensively, were made by 

 Mr. E. R. Long in 1915. Imbibition was tested by noting the increases 



12 p.m. 



12 p.m. 



12 p.m. m 



Fig. 28. — Portions of auxographic record of variations in length of a joint of Opuntia from October, 

 1915 to October 1916. The joint had formed roots which depended in a glass jar of tap- water 

 and the joint was held firmly in a natural erect position. The preparation stood in the south 

 end of a glass house and was exposed to the alternations of sunlight and darkness with accom- 

 panying temperatures as low as 8° C. in the morning and as high as 45° C. in midsummer 

 afternoons. The actual variations are amplified 30 and elongation is denotec^ by the down- 

 ward movement of the pen. Divisions of scale =1 cm. The joint was in a stage of com- 

 pleted growth during the week beginning October 18, 1915, at which time elongation occurred 

 from evening until the middle of the forenoon, and shortening during the remainder of the 

 day, with the length less at the end of the week than at the beginning. The records for Murch 

 and April show a similar daily periodicity, but with an increase in length which may be 

 ascribed to imbibition under the advancing temperatures. The daily losses and gains are 

 more nearly equal in May and June and the variations of narrow amplitude. Some loss is 

 shown in July and the reactions of the joint a year from the beginning of the record show a 

 daily variation different in many particulars from those of the previous year. See figure 29 

 for a continuation of the record. 



