THE EFFECT OF SLOPE EXPOSURE. 211 



In this connection an interesting point may be mentioned. 

 On the northern slopes the trees of C. spicata are being attacked 

 and killed by white ants, and once a tree has fallen, it is very 

 soon completely destroyed. On the southern slopes the trees are 

 not attacked, and apparently even the broken stumps must be 

 immune, seeing that they are still able to give rise to strong, 

 vigorous coppice shoots, capable of growing into healthy young 

 trees. Typical white ants' nests are absent from the southern 

 slopes, and, while one cannot assume from this a complete absence 

 of termites, the species on this side must be different from that 

 on the other. Associated with the difference in vegetation, there- 

 fore, there is a difference in the animal population of the two 

 slopes. 



In order to determine some of the climatic differences, which 

 are responsible for the production of such marked divergences in 

 the character of the vegetal covering, it was decided to measure 

 the following factors throughout the course of a single day: — 



1. Sunlight intensity. 



2. Eate of evaporation. 



3. Air temperature. 



4. Soil temperature. 



5. Soil moisture content. 



In addition, since Cussonia spicata occurs on both sides of the 

 hill, the rate of transpiration of two trees, one on each side, was 

 followed throughout the day. 



Two stations were chosen, one on each side and as nearly as 

 possible at the same height, though the one on the north side was 

 somewhat lower than the other. That on the north side was in 

 open grass land near a typical Cussonia clump. For the southern 

 station a space was cleared — in order to remove the shading effect 

 of the tall shrubs — near a tree of Cussonia spicata within the area 

 of developing bush. At each station two of the senior students 

 from the Natal University College were in charge of the observa- 

 tions, and I should like to take this opportunity of thanking these 

 gentlemen for the willing assistance which they rendered in this 

 connection. 



Eesults. 



1. Sunlight Intensity. This was measured by a chemical 

 method depending on the amount of iodine set free in a mixture 

 of potassium iodide and sulphuric acid in the presence of sunlight. 

 This method has been successfully used by one of our students 

 for the measurement of light intensity in various plant habitats, 

 and will be described in detail in a subsequent paper. For the 

 present it is sufficient to say that bottles containing similar 

 quantities of the mixture were exposed for periods of one hour 

 each, and the amount of iodine released was determined by 

 titration. Results are given as mgms. of iodine set free per hour 

 and are tabulated in the accompanying table. 



