8 SYLVICULTURE IN THE TROPICS 



PT. I 



readily unless prevented from doing so by an imperme- 

 able substratum of clay. They are the least fertile of 

 soils, therefore the number of species which can grow 

 on them is also limited, especially where the rainfall 

 or other supply of moisture is scanty. In such places 

 the soil loses its cohesiveness and stability, plants find 



PlG. 2. Mixed forest on limey soil, Sudan. 



no anchorage in them, and, moving in front of prevailing 

 winds, the sands advance in billows, which are a great 

 danger to cultivation. 



AVhere, however, a sufficient amount of moisture and 

 cohesiveness is given to the soil, certain plants are able 

 to spring up and exist, and among them such trees as 

 the umbrella-shaped Acacia tortilis, although, as Fig. 

 3 shows, their hold on the soil is not very great, and 



