Chap. II] PERIODIC PHENOMENA IN THE TROPICS 243 



humid districts is an illusion. To the attentive observer repose is manifest 

 everywhere side by side with activity, and both these conditions continually 

 interchange like the parts of a kaleidoscope. The difference between such 

 a vegetation and one exposed to the influence of alternately favourable and 

 unfavourable seasons is confined to the fact, that in the constantly humid 

 district the sum of repose and activity remains approximately constant in 

 every season, whilst in districts with alternating seasons it periodically 

 increases and decreases. 



2. PERIODICITY IN THE VEGETATIVE DOMAIN, 

 i. LEAF-FALL. 



In spite of numerous travellers' reports to the contrary, the notion still 

 prevails that tropical forests are for the most part evergreen, and essentially 

 occupy constantly moist districts only, whereas districts with markedly dry 

 seasons are supposed to produce forest growth only along the margins 

 of water-courses. This incorrect idea is connected with the no less 

 incorrect assumption that dry seasons are unsuitable for forests. 



In reality the case has quite a different aspect. The tropical forest is for 

 the most part composed of periodically leafless trees, and, as has already been 

 shown 1 , is by no means excluded from districts with very dry and hot 

 seasons. Indian forest-botanists, especially Brandis and Kurz, were the 

 first to make us acquainted with the distinctive characters of tropical 

 forests that are evergreen and forests that are green in the rainy season. 

 Woodland in districts with periodic and well-marked dry seasons has 

 during the dry season a greater resemblance to Central European woodland 

 during winter than to the dense luxuriant rain-forests which are usually 

 regarded as the typical tropical forests. The picture of repose, however, 

 that is evoked by the dry season, is much less uniform than that of a winter 

 landscape in temperate zones. 



Thus, in many cases, woodlands near the water retain all their foliage or 

 a large part of it during the dry season, and form green strips and spots 

 on the landscape which is otherwise mainly brown and grey in tint. In 

 addition, outside the reach of water-infiltration, differences in the chemical 

 and physical nature of the soil affect the time and extent of the leaf-fall. 

 A greater amount of moisture in the soil delays defoliation and accelerates 

 the opening of the leaf-buds. Warming has also observed that woody 

 plants growing on a calcareous soil become barer of foliage than on other 

 kinds of soil. 



Specific differences in woody plants also come into play. Under identical 



1 See p. 165. 

 R 2 



