424 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



conditions, although in most of them it is tending to diminish 

 rapidly in area owing to the activities of Man, and in some consider- 

 able tracts has disappeared altogether. Replacement is mainly by 

 secondary growth on areas of cultivation. In addition, subtropical 

 rain forests (which seem best treated here) occur widely in central 

 and southern South America, around the Tropic of Cancer in Central 

 and North America and in eastern China, farther north than the 

 tropical rain forest in the Himalayan region and farther south in 

 East Africa, and also in Hawaii and southeastern Australasia. 



The country occupied by tropical rain forests is usually flat or 

 rolling, though they may extend up the lower slopes of mountains 

 to an altitude of about 1,000 metres (3,281 feet) or even higher. 

 In some areas rain falls almost every afternoon and night practically 

 throughout the year, in others there are one or two dry seasons^ of 

 not more than three months each. Often the rain will pour down 

 for days or weeks, and everything becomes soaked in a thick grey 

 mist. The temperature is relatively high and uniform, the annual 

 means being normally around 25-26° C, and the rainfall commonly 

 totals between 200 and 400 cm. annually, though in places there 

 may be much more. The relative humidity also tends to be high, 

 being usually above 80 per cent., though comparatively low values 

 may obtain for short periods. Some notion of what the climate 

 is like may be obtained from the tropical palm houses of botanical 

 gardens. But although the light is dazzling when the sun shines 

 on the upper canopy from its midday position high in the sky, 

 beneath the commonly three stories of trees a sombre gloom prevails, 

 the atmosphere being humid and close. Nevertheless some rays 

 may penetrate and sun-flecks prevail — -and, it seems, be micro- 

 climatically and physiologically important. 



In these tropical rain forests it is chiefly in the tree-canopy that 

 animal life flourishes — of innumerable and sometimes gaily-coloured^ 

 Insects, Tree-frogs, Lizards and Snakes, Birds, Squirrels, Monkeys, 

 and so forth, many of which never touch the ground during their lives 

 (I. V. Polunin voce). The component plants may lose their leaves 

 individually each year or so ; but there is no regular seasonal change 



^ Professor Paul W. Richards points out {in lift.) that although a dry season cannot 

 be adequately defined in tern^s of months with less than a certain minimum rain- 

 fall, dry seasons in these regions may be considered as consisting approximately 

 of those months having less than 4 inches (about 10 cm.) of rain. 



^ In general, however, protective coloration is more characteristic of rain-forest 

 fauna — particularly with the Insects, which tend to be brown or green and to 

 harmonize with their environment (J. A. R. Anderson and I. V. Polunin in lift.). 



