SECTION I. THE TROPICAL ZONES 



CHAPTER 1 



GENERAL CHARACTERS OF THE TROPICAL CLIMATE 

 AND ITS EFFECTS ON VEGETATION AND FLORA 



i. General Characters of the Tropical Climate. i. Atmospheric Precipitations. 

 Rain. Relative humidity. Clouds, ii. Heat. Temperature of the air. Heating by 

 direct insolation, iii. Light and the Ultra-violet Rays. Intensity of the chemical rays of 

 light. 2. Some General Effects of the Tropical Climate on Plant-life. i. Pro- 

 cesses that are chiefly influenced by Heat. Cardinal points. Cases of rapid and slow 

 growth. Transpiration in sun and shade, ii. Physiological Action of Tropical Light 

 on Plants. Protective measures against intense light. Decomposition of chlorophyll. 

 Position of foliage-leaves. Photic ration of shade-plants, iii. Physiological Action of 

 Atmospheric Precipitations on Plants. The controlling influence on the character of the 

 vegetation and the periodic processes. Ombrophily and ombrophoby. 3. The Floristic 

 Character of the Tropical Zone. Survey of the megathermic group of forms. 



i. GENERAL CHARACTERS OF THE TROPICAL CLIMATE, 

 i. ATMOSPHERIC PRECIPITATIONS. 



The amount of the annual rainfall in the tropics varies between 5 meters 

 and more at some mountain-stations and a few centimeters in desert 

 districts. It is on the average greatest in the equatorial zone, that is to 

 say between 5 N. and 5 S., and it diminishes more rapidly in a northerly 

 than in a southerly direction. The desert districts within the tropics 

 belong with few exceptions to the border zones and merely represent 

 the tropical continuations of the extensive subtropical deserts. 



At least as important to plant-life as the amount of precipitation is 

 its seasonal distribution. The year in the greater part of the tropical 

 zone consists of a dry season, usually coinciding with the winter months, 

 and a rainy season in the summer months. Some equatorial districts, 

 for example Guiana, possess two unequally long rainy seasons, and some 

 others, for example Singapore, have no distinct rainy season in the year. 

 The difference between rainy and dry seasons is more decided inland, 

 especially in plains, than along the coasts and on mountains, where its 

 influence on plant-life is frequently no longer appreciable. The coin- 

 cidence of the rainy and dry seasons with certain months in the year 

 is not a matter of physiological importance to the vegetation, except in 

 some border districts of limited area, since the differences between the 

 temperatures in winter and summer are usually very small. 



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