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PLANT GEOGRAPHY 437 
dense and there is little development of undergrowth and ground 
covering. Here the trunks of the trees stand out as gigantic 
columns. The great luxuriance of tropical rain forests is ex- 
plained by the fact that moisture, temperature, and light are 
continuously favorable. The same favorable conditions, particu- 
larly in regard to moisture, are responsible for the great develop- 
ment of epiphytes. Large epiphytes are more dependent on a 

Fig. 491. View in rain forest on lower slope of Mount Maquiling, 
Philippine Islands 
Note the density of the foliage, which hides the large tree trunks 
e 
continuous supply of atmospheric moisture than are terrestrial 
plants, and they cannot stand prolonged periods of adverse 
moisture conditions, whether these are due to lack of atmospheric 
moisture or to cold. In keeping with the great luxuriance of 
the vegetation, the number of species of trees, vines, and epi- 
phytes in moist tropical forests is greater than in less favored 
regions. More than a hundred species of trees have been counted 
on a quarter of a hectare. 
