Imbibition and Growth of Opuntia. 



133 



TABLE 105. 



The material to which the high hydration capacity of Opuntia is due 

 is the mucilage which exudes from the damaged cells of cut surfaces 

 of the parenchymatous tissues. This is included among the pentosans 

 together with agar and various gums, including tragacanth, acacia, 

 cherry gum, and prosopis gum, many of which have a high hydra- 

 tion capacity and infinite dispersion hi water. 



The manner of the formation 

 of this material is a matter of 

 no little importance in connec- 

 tion with varying growth and 

 hydration capacity. Briefly 

 stated, the depletion of the 

 water-content of a cell accel- 

 erates the conversion of poly- 

 saccharids with low hydration 

 capacity into pentosans (muci- 

 lages and gums) which have 

 an extremely high water capac- 

 ity, and it is this change which is followed by the large swelling of 

 tissues in certain stages or in dry seasons. The transformation in 

 question is not reversible. While some decrease hi these pentosans 

 may occur, it is not known by what steps it takes place. 1 



The composition of etiolated plants presents proportions of the 

 principal constituents different from those of the green plant, and if 

 one of the main conclusions of the present work is valid, they might 

 be expected to exhibit some water-relations different from those of 

 green plants. 



Some shoots of Opuntia which had developed in a dark room at 

 Carmel in equable temperatures below 20 C. were available in July 

 1917, and these displayed some singular temperature effects. The 

 stems were from 20 to 30 cm. long and compressed ovate in cross- 

 section. Short sections were cut out and immersed in dishes in the 

 usual manner. Most of the development had been at 16 to 18 C., 

 and some of the shoots were placed in a second chamber at 30 to 

 31.1 C. for 2 days before being swelled. The illumination of a 

 small incandescent bulb used in obtaining these measurements had 

 given the stems a slight greenish tinge, but the energy derived from 

 this source was not sufficient to have caused any serious change in 

 the composition of the shoot. Sections were about 5.5 mm. in dia- 

 meter, and sets of three were selected for testing under the auxo- 

 graph, so that the older basal parts of the stem and the newest 

 apical portions were represented in the separate measurements. One 



1 See MacDougal and Spoehr. The origination of xerophytism. The Plant World, 21 : 245. 

 1918. Also MacDougal, Richards, and Spoehr. The basis of succulence in plants. Bot. Gaz., 

 67:405. 1919. 



