328 MACDOUGAL AND SPOEHR— GROWTH AND IMBIBITION. 



products such as acids, the presence of which may cause accelera- 

 tion, retardation or cessation of growth by modifying imbibition or 

 capacity for absorption of water. 



It is obvious that a determination of the water-absorbing capac- 

 ity or swelling coefficient of a growing organ would be an index of 

 its capacity for enlargement at that moment, and by the use of dif- 

 ferential solutions the influence of acidity or alkalinity on the process 

 may also be ascertained. The catabolic and synthetic processes 

 which accompany growth are in the main continued in mature 

 organs, especially if these contain tracts of open meristem as do the 

 joints of Opuntia. It was thought highly important therefore to 

 make extensive tests of the swelling capacity of Opuntia with 

 analyses of the carbohydrate content of the joints. These tests 

 yield some data of great interest when considered in connection with 

 the growth records given in the preceding section of this paper. 



The flattened joints of Opuntia sp. which formed the prin- 

 cipal experimental material are elongated oval in outline, the basal 

 part being usually about 20-24 mm. in thickness and the apical 

 part half or less than half of this diameter. After some extensive 

 comparisons of sections from all parts of the joint it was found that 

 the apical third of the member furnished the best material for com- 

 parative purposes. Sections or disks about 12 to 14 mm. across 

 were cut from this region with a cork borer, avoiding the inclusion 

 of nodes bearing the spines and spicules. Such sections consisted of 

 the indurated epidermal layers between which was a cylindrical 

 mass of parenchymatous cells, the outer ones being chlorophyllous 

 and some of the inner ones being mucilaginous. An anastomosed 

 network of thin fibrovascular strands was included in the paren- 

 chymatous mass and this mechanical tissue probably checked expan- 

 sion in some cases, especially those in which disks were taken too 

 close to the nodes. More care was exercised in this matter in 1917 

 than in the preceding tests, a fact that may be taken to explain in 

 part at least the decreasing number of anomalies as the work pro- 

 gressed. Three of such disks about 12 mm. across the epidermal 

 surfaces and from 6 to 1 1 mm. in thickness were arranged in a tri- 

 angle in the bottom of a stender dish and a triangle of thin sheet 



