Chap. I] CHARACTERS OF THE TROPICAL CLIMATE 225 



iii. PHYSIOLOGICAL ACTION OF ATMOSPHERIC PRECIPITATIONS 



ON PI A NTS. 



The differences in the oecology of tropical plants are in the first place 

 associated with differences in the atmospheric precipitations. These 

 alone, usually without any, even indirect, co-operation on the part of the 

 temperature, cause the domination of woodland or of grassland, the 

 hygrophilous, tropophilous. or xerophilous character of the vegetation, 

 and the periodic phenomena. Accordingly, the differences in the climatic 

 humidity also determine the range of species within the tropics. 



A separate chapter is devoted to the various types of tropical wood- 

 land and grassland arising from differences in the atmospheric precipita- 

 tions, and to the climatic conditions determining their presence, and also 

 to the periodic phenomena. 



Reference may here be made to Wiesner's investigations regarding the ombvophily 

 and onibrophoby of tropical vegetation, for they are as yet too incomplete to be 

 serviceable in characterizing the several climatic districts. According to Wiesner, 

 the great majority of plants in the moist hot western parts of Java possess markedly 

 ombrophilous foliage, and this condition may probably extend to the vegetation in 

 a constantly moist tropical climate. On the other hand, in those parts of the torrid 

 zone where long rainless periods regularly prevail, the condition of affairs may be 

 changed in favour of ombrophobous species. Investigations on this subject have not 

 been conducted, but the frequency in such districts of succulent species and other 

 plants with unwettable coatings favours this opinion. 



Nevertheless, many decidedly ombrophobous plants thrive in a constantly moist 

 climate ; thus Wiesner rightly draws attention to the fine growth of various species 

 of Opuntia and Cereus in the botanic garden at Buitenzorg. These plants are, how- 

 ever, confined to very open sunny situations, where strongly ombrophilous plants 

 would suffer from too great transpiration. On the other hand, the feeble growth 

 in West Java and other very moist tropical districts of many plants from a drier 

 climate is to be attributed to onibrophoby. This has been proved by Wiesner 

 particularly in regard to roses, which nearly always assume a very stunted form 

 in tropical gardens. Their foliage is weakly developed and caducous, their corollas 

 are small and irregular. The different kinds of roses are, however, unequally 

 ombrophobous, and consequently display different degrees of deficiency, or may 

 absolutely refuse to grow. 



To the indigenous ombrophobous plants of constantly humid districts there belong 

 in particular those with delicate pinnate leaves, which by their movements more 

 or less effectively evade the violence of the rain, such as Mimosaceae, species of 

 Oxalis, and the like. But such forms are scantily represented in Java, especially 

 in the forest ; their abundance is characteristic of the vegetation of drier climates. 



In accordance with its ombrophily, the foliage in a constantly humid climate is as 

 a rule easily wettable ; besides, as Wiesner shows, foliage that is wetted with difficulty 

 becomes easily wettable when exposed to continuous humidity, and the reverse is 

 the case. Young leaves are as a rule unwettable and ombrophobous ; later they 



SCHIMI'ER ( ) 



