DEPARTMENT OF BOTANICAL RESEARCH. 77 



Plant Physiology of the Johns Hopkins University. Simultaneously, 

 and with similar plants, the diurnal march of the permeabihty of the 

 leaves to an-flow under pressure was also studied, by means of the F. 

 Darwin and Pertz porometer. This instrument is calculated to measure 

 the degree of stomatal opening from time to time, and if this feature 

 is indeed the controlhng condition in its operation, then the results 

 obtained should be capable of interpretation to show the diurnal march 

 of stomatal diffusive capacity in the sense of Brown and Escombe. 



The results obtained from these parallel series of observations are 

 similar in that both methods agree in showing the same kind of daily 

 march, the values of both indices rising to a maximum in the day and 

 falling to a minimum in the night. The range of variation, however, 

 between the minimum and maximum is generally somewhat greater 

 for the indices of stomatal diffusive capacity (derived from porometer 

 readings) than for those of transpiring power (derived from the hygro- 

 metric paper tests) . It appears that the porometer readings do furnish 

 data for deriving the stomatal diffusive capacity, at least in these 

 Zehrina leaves, but that this diffusive capacity can not be considered 

 as quite proportional to transpiring power, as has been pointed out 

 from earher studies carried out in cooperation with the Desert Labora- 

 tory and as has been emphasized by Renner, conditions other than that 

 of stomatal diffusive capacity are surely influential in determining foliar 

 transpiring power. It is clear, however, that stomatal capacity is the 

 main condition influencing transpiring power in such leaves as were here 

 used. 



It should be emphasized in this connection that transpiring power 

 and the actual rate of transpiration are not at all the same thing; the 

 former represents simply the group of internal conditions influencing the 

 latter rate, vv'hich is of course greatly influenced also by a group of 

 external conditions, such as the evaporating power of the air, etc. 



The Autonomic Movements and Water-Relations of Cacti, by Edith B. Shreve. 



The results obtained in previous years on movements of stems of 

 Opuntia versicolor have been further tested and extended to other 

 species of cacti. The data have been divided into two main parts, the 

 first of which traces the causes of the movements as far as changes in 

 turgidity, which are in turn due to periodic differences between water- 

 intake at the roots and water-loss by transpiration ; the second includes 

 extended measurements of transpiration and of water-intake, which 

 were undertaken with the aim of finding the causes of the variations 

 in these phenomena. The first part has been made ready for publi- 

 cation and contains the following summar}^: 



(1) Ten species of Opuntia and also Carnegiea gigantea have been 

 found to show seasonal movements of the branches, which consist of a 

 drop during desiccation and a rise during subsequent recovery, and 

 these movements have been correlated with turgidity changes. 



