70 



CARNEGIE INSTITUTION OF WASHINGTON. 



less, of the coast in the Namib. This is shown at and near Swakopmund. 

 With an average precipitation of 0.65 inch, the rate of evaporation at 

 Swakopmund in winter and spring is surprisingly low. This is affected in a 

 marked manner by the direction of the winds and by fog. Thus for four 

 days ending July 13 a water-loss of 342 c. c. was recorded. For a portion of 

 this time a land wind from across the arid hinterland (Namib) was blowing. 

 At another time the atmometer lost only 132 c. c. in eleven days during a 

 period of fog. And, finally, the total water-loss of September and October 

 was only 1,581 c. c, which, it will be seen, was about the same as at Grahams- 

 town with a precipitation amounting to 28.79 inches. Sparse as it is, the 

 vegetation of the western Namib would not be so abundant if it were not for 

 the marked and direct effect on evaporation of the fog. 



On the Transpiring Power of some Karroo Plants in Winter and Spring, 



by W. A. Cannon. 



The rapidity with which desiccated strips of paper impregnated with 



cobalt chloride change color when placed on the surface of leaves as compared 



to the rate of change over a free water-surface, the index of transpiring power 



of Livingston, 1 was observed in several native and introduced plants in 



certain of the more arid parts of South Africa, especially in the Great Karroo, 



in late winter and in spring of 1921 . The growth-habit of the species examined 



included herbaceous forms, annuals and perennials, as well as shrubs and 



trees. The ascertained indices of transpiring power 



Rate of color change over water .. 



= e/t 



Rate of color change on leaf 



for the species examined showed a considerable range of variation as between 

 species, and in the same species, but nevertheless is relatively low, as appears 

 in table 2. 



Table 2.- 



-Maxima and minima indices of transpiring power of Karroo plants, July to 



October 1921. 



*Average of maximum and of minimum of both leaf-surfaces. 



Although the studies were carried on during only a portion of the year, 

 they suggest several features of ecological import, some of which can be 

 referred to in this place. Of the species given above, the Aloes, Cotyledons, 



1 B. E. Livingston, The resistance offered by leaves to transpiration water-loss, Plant World, 

 vol. 16, p. 1 (1913). 





