Chap. I] CHARACTERS OF THE TROPICAL CLIMATE 215 



2. SOME GENERAL EFFECTS OF THE TROPICAL CLIMATE 



ON PLANT-LIFE. 



i. PROCESSES THAT ARE CHIEFLY INFLUENCED BY HEAT. 



It appears from the foregoing that the tropical climate differs from that 

 of higher latitudes chiefly in its uniform and high temperature, and in the 

 greater activity of the rays of heat and of light. The atmospheric pre- 

 cipitations exhibit neither in intensity, nor in their sequence in time, any 

 essential difference as compared with the temperate zone, where at certain 

 stations as great a rainfall occurs as at the rainiest points in the tropics, 

 and where extensive districts exhibit a similar alternation of dry and 

 rainy seasons. That, notwithstanding this, precipitations have still greater 

 influence on the oecology of tropical plants than on temperate ones 

 and evoke in them a series of characteristic peculiarities, is partly caused 

 by the combination of greater heat with greater humidity, and partly 

 by the fluctuation of the humidity in contrast with the steadiness of 

 the heat. 



Owing to the great uniformity and considerable height of the tempera- 

 ture in the tropics, much smaller differences in the harmonic optima, and 

 consequently a much greater uniformity in the curve of temperature showing 

 the oecological optimum 1 , are to be expected, than in higher latitudes. 

 More precise data on this matter are not at present available, since the 

 physiological cardinal points as well as the oecologically most favourable 

 degrees of temperature have as yet been determined only for temperate 

 plants, in which, corresponding to the natural conditions, they lie far apart. 

 We can therefore say no more upon this subject, for it is inadmissible 

 to draw conclusions regarding the cardinal points of vegetation in the 

 tropics merely from the extreme temperatures of the air, since the nocturnal 

 cooling due to radiation, which in the dry season considerably exceeds 

 that of the air, as well as the strong heating by direct insolation, must 

 play an all-important part in many physiological processes. 



Growth. 



Among the physiological processes with a high optimum of temperature, 

 growth, at any rate after the period of germination, takes a prominent 

 place. It would be instructive to institute comparisons between plants 

 of one and the same species in the tropics and in temperate zones, 

 under external conditions otherwise as similar as possible. Up to the 

 present only very few observations regarding the rate of growth of 

 tropical plants are available, and from these only one conclusion can be 



1 See p. 44. 



