Chap. Ill] TROPICAL WOODLAND AND GRASSLAND 279 



great forests, possibly rather as fringing forests, clothe the plains at the mouth of the 

 Congo (9°4'S., 12° 15' E.), whilst the highlands on the lower Congo (San Salvador 

 6° 17' S., 1 4 53' E., with rainfall of 998 mm.) are covered with savannah. South of 

 the mouth of the Congo, again, we at once find savannah with gigantic Adansonia. 

 In the southerly direction the savannah becomes continually poorer, and at Kinsembo 

 a desert character is assumed, where succulent plants like Euphorbia and Aloe 

 predominate. North of Mossamedes (15 11' S., 12 8' E.), Welwitschia mirabilis 

 first shows itself, the grass vegetation becoming constantly scantier, and beyond the 

 Kunene (i7°3o' S., 14 E.), the river bounding Lower Guinea, only scattered tufts of 

 grass and accommodating thorn-plants grow on the parched and stony ground. 



In the preceding remarks, the distribution of high-forest — possibly 

 rain-forest throughout — and of savannah is shown in the clearest manner 

 to be dependent on the amount of rainfall. Everywhere high-forest alone 

 predominates when the rainfall attains 200 cm. in the year, but is entirely 

 supplanted by savannah when the rainfall does not exceed 170-180 cm. 

 Finally savannah gradually dwindles into desert at a rainfall of 30-35 cm. 



Here and there in fertile districts, especially where there is much 

 atmospheric humidity, savannah alternates with savannah-forest, which 

 appears to be absent from other districts, for instance the high plateau 

 on the Congo and to the south of the mouth of the Congo. 



Should the inquiry be made, why districts that are too dry for high- 

 forest are occupied by savannah, which is grassland, and not by scrub, 

 we may reply that the frequency of the precipitations shown by the great 

 number of rainy days, as well as the not very high temperature, are 

 favourable to grassland. Pechuel-Losche mentions the by no means rare 

 occurrence of prolonged periods of drought on the Congo coast, for 

 instance years with only 20 centimeters of rainfall, and, as has been 

 explained already, such droughts are much less fatal to the continued 

 existence of grassland than to that of woodland. The heavy dew of the 

 normal dry season will possibly continue during these periods of drought 

 to moisten the surface of the ground, but is of no value to the roots of 

 woody plants. Our meteorological table for San Salvador, on the 

 savannah-clad highlands of the lower Congo, shows that strong winds 

 prevail there, especially during the dry season. This circumstance, 

 unfavourable to woodland, favours grassland. Our present knowledge 

 of the African climate is, however, still too incomplete to afford a com- 

 pletely satisfactory answer to the question postulated. 



Our meteorological knowledge of the savannah districts on the elevated 

 plateau of central tropical East Africa is still very incomplete, but the 

 available data, for instance the tabular statement given below, exhibit 

 a typical savannah climate, with a moderately hot vegetative season, during 

 which the rainfall is about 100 centimeters in very frequent showers, as the 

 number of rainy days tells. 



