DEPARTMENT OF BOTANICAL RESEARCH. 69 



A comparison of the average precipitation for September-October at these 

 stations with the observed evaporation for the same months in 1921, 



precipitation p 

 evaporation e 



gives a measure of their relative aridity which is of value. Thus, at Beaufort 

 West the average September rainfall is 0.52 and that for October is 0.99 

 inch. The total evaporation for each of these months, 1921, was found to 

 be 1,414 and 1,675 c. c. respectively. The precipitation-evaporation ratios 

 (p/e) for Beaufort West are 0.000367 September and 0.00021 October. The 

 average rainfall at Matjesfontein for September is 0.29 inch and for October 

 0.36 inch. The total evaporation for the two months, 1921, was found to be 

 902 and 1,650 c. c, respectively. The p/e ratios, therefore, for September and 

 October at Matjesfontein are 0.00032 and 0.00021. At Grahamstown the 

 average precipitation for September and October is 3.28 and 3.1 inches. The 

 evaporation for the two months, 1921, was 754 and 823 c. c. The p/e ratios 

 for the two months are, therefore, 0.0042 and 0.0037. The relative aridity 

 of the Great Karroo in spring is further indicated by a comparison of the 

 p/e of Beaufort West, Pietermaritzburg, and Kirstenbosch for November. 

 Of these stations the annual rainfall at Kirstenbosch (Bishop's Court, Cape 

 Town), is about 52 inches, of which about 80 per cent occurs in the cool 

 seasons. The average precipitation for November at each of the stations is 

 as follows: Beaufort West, 0.75 inch; Pietermaritzburg 5.7 inches; and Kirs- 

 tenbosch (Bishop's Court), about 2 inches. The total evaporation in 

 November 1921 was found to be 1,216 c. c. at Beaufort West, 955 c. c. at Pieter- 

 maritzburg, and 927 c. c. at Kirstenbosch (National Botanic Garden). The 

 November p/e ratios, therefore, are as follows: at Beaufort West, 0.00061; 

 at Pietermaritzburg, 0.00576; and at Kirstenbosch, 0.0021. These ratios 

 indicate the march of the seasons with the accompanying characteristic 

 changes in aridity, not only in the well-watered regions, but also in those not 

 thus so well favored. And they reveal in a striking manner the relative 

 great aridity of portions of the Great Karroo. 



The obtained results point also to the possible importance, as influencing 

 the evaporation-rate and hence as having noteworthy effect on plant life, of 

 such factors as prevailing winds, as the influence of neighboring highlands 

 or regions of greater or less moisture, and as the effect of ocean fog. The 

 relation of certain such secondary environmental factors can be briefly re- 

 ferred to here. They are of especial importance in regions of little precipita- 

 tion. Thus, in the Great Karroo the kind as well as the abundance of the 

 vegetation is often associated with aspect. This feature is well exemplified by 

 the vegetation of isolated small hills, or kopjes, which may be unlike on the 

 north and on the south sides. Atmometer readings on the sides of the kopjes 

 toward more arid or warmer regions, or on the warmer side of the kopjes, 

 are noticeably higher than on the opposite aspects, and these results appear 

 to be in good correspondence with the relative xerophylly of the vegetation 

 of the two sides. Again, the relatively abundant vegetation in portions of the 

 Great Karroo where the rainfall is small is probably to be associated with a 

 fairly high humidity of the air due to winds from contiguous regions of higher 

 precipitation. Analogous conditions obtain with 50 kilometers, more or 



