HAP. I] CHARACTERS OF THE TROPICAL CLIMATE 225 



iii. PHYSIOLOGICAL ACTION OF ATMOSPHERIC PRECIPITATIONS 



ON PLANTS. 



The differences in the oecology of tropicil plants arc in the first place 

 jsociatcd with differences in the atmospheric precii)itations. These 

 one, usually without anw even indirect, co-operation on the part of the 

 ■mperature, cause the domination of woodland or of grassland, the 

 ygrophilous, tropophilous, or xerophilous character of the vegetation, 

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

 iimidit)' also determine the range of species within the tropics. 

 A separate chapter is devoted to the various tN^pes of tropical wood- 

 nd and grassland arising from differences in the atmospheric precipita- 

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

 ) the periodic phenomena. 



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

 id ombrophoby of tropical vegetation, for they are as yet too incomplete to be 

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

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

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

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

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

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

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

 ants with unwettable coatings favours this opinion. 



Nevertheless, many decidedly ombrophobous plants thrive in a constantly moist 

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

 Opuntia and Cereus in the botanic garden at Bnitenzorg. These plants are, bow- 

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

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

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

 mate is to be attributed to ombrophoby. This has been proved by Wiesner 

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

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

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

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

 solutely refuse to grow. 



To the indigenous ombrophobous plants of constantly humid districts there belong 

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

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

 ;a!is, and the like. But such forms are scantily represented in Java, especially 

 the forest ; their abundance is characteristic of the vegetation of drier climates, 

 n accordance with its ombrophily. the foliage in a constantly humid climate is as 

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

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

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



