294 THE FLORA OF BRITISH EAST AFRICA 



expanding above like an umbrella. This form is apparently 

 acquired as a protection against the tornadoes that sweep 

 over them. The shape is not much use to an isolated 

 tree, for the wind can then get underneath the branches and 

 tend to wedge it up ; but these trees usually grow packed 

 together, and their flat tops form a surface, over which the 

 storms sweep with little effect. 



It is no doubt also as a protection against the wind that, 

 as if to atone for the leaflessness of the trees, the flowering 

 plants are woody. Thus on the plateaux we have to look for 

 the main masses of blossom, not to the turf on the meadows, 

 but to the clumps of shrubs, which often glow with the yellow 

 of the Doinbeya, the Calodendruvi, and the St. John's Wort 

 {Hypericuvi), or appear gray with the groundsel-like flowers of 

 the " lelesha " {Tardionanthiis camphoratus, L.) 



In spite, however, of such points as these, the flora is mainly 

 of interest in connection with the problems of geographical 

 distribution. For the contrast between the tropical flora of the 

 coastal plain or the Tana delta and the temperate floras of the 

 Alpine zone, between the dense jungles and forests of the 

 lower slopes of Kenya and the sparse scrub of the sandy 

 Nyika, is the most striking feature in the natural history of the 

 region. To a geologist this is of especial interest, for the 

 distribution of the plants is determined to a considerable extent 

 by the geological structure of the country. The nine zones 

 are no doubt largely due to the different temperatures ot 

 different heights. But altitude is not the only factor ; the 

 boundary lines of the zones do not exactly follow the contours. 

 The variations of temperature due to height are overpowered by 

 the influence of the amount of moisture in the air, which is 

 often determined by the character of the rocks in the country 

 to windward. Thus the western sides of Kilima Njaro and 

 Kenya have a greater rainfall and broader snowfields than the 

 eastern, as the lava country to the west is more fertile than 

 the sandy wastes to the east. The geological factors, more- 

 over, not only determine the amount of the rainfall, but whether 

 this is used to the best advantage. The higher mountains, and 

 the lowlands beside the coast and the larger rivers, have a 

 luxuriant vegetation, because the former intercept moisture 

 from the elevated currents of air, while the latter are watered 



