DEPARTMENT OF BOTANICAL RESEARCH. 



71 



The acidity of the tissues is due to certain modifiable features of 

 respiration. Acidity of plants which have undergone long-continued 

 desiccation and starvation is low, since the amount of carbohydrate 

 from which they are derived has been decreased. 



Katabolism in extended desiccation and starvation e^'entually breaks 

 down the plasmatic colloids and includes hydroh'sis of the cell-walls 

 of the cortex. 



The loss in weight of Opuntia in full sunlight and in diffuse light is 

 not very different during the first 35 days of exposure, and is practically 

 the same after that length of time. The position of the flattened 

 joints in the open may modify the rate of loss. 



Opuntia desiccating in the open shows an increase in dry weight, but 

 a decrease in hydrolyzable carbohydrate, while the acidity is not 

 markedly different from the normal, though slightly less. Desiccation 

 in diffuse light results in increase of acidity, increase of dry weight (not 

 as pronounced as in the open), and decrease in hydrolyzable carbo- 

 hydrate. 



Reversible Changes of Form in Succulents, by D. T. MacDougal. 



Gross measurements of joints of platyopuntias during their develop- 

 ment and in succeeding seasons shovv^ed that these bodies were subject 

 to reversible changes in length and width, as exemplified by the 

 accompanying figures obtained from a joint of Opuntia discata (No. 3) 

 at Tucson, in 1912, 1913, and 1914 (shortening indicated by bold-face 

 figures) : 



A number of mature joints of this succulent w^ere placed in bearing 

 with precision auxographs of a type devised by the author in 1902, 

 in which both expansions and contractions, magnified 20 to 50 times, 

 are recorded. The changes of size of a mature joint, such as those 

 indicated in ]\ larch 1913, consist mainly of swelling by increased absorp- 

 tion of water made possil^le b}^ disintegration of acids in the sap. This 

 does not proceed at an even rate, but is most rapid in the first half of 

 the day, being greatest from 11a. m. until 2 p. m. During the remain- 

 der of the day this action would fall off and actual shortening might 

 occur at night as a result of increased acidity and heightened transpira- 

 tion. These reversible changes in form also take place in young joints. 



