RAIN-FOREST AND DESERT MOUNTAINS 139 



In the Blue Mountains there is a difference of physical condi- 

 tions between the floor of the forest and its canopy which is 

 sufficiently great to be designated '^ith all propriety as a differ- 

 ence of climate. Instrumental measurement of the daily curves 

 of temperature and humidity on the floor of the rain-forest and 

 in its canopy has been made in connection with a study of the 

 factors controlling the distribution of the filmy ferns. ^ The 

 difference between the highly uniform conditions of temperature 

 and the steadily high humidity of the lowest portion of the 

 rain-forest, and the greater amplitude of both these factors in 

 the tree tops, is undoubtedly exceeded in a striking degree in the 

 loftiest lowland rain-forests. Such difference is paralleled by 

 some of the structural features of the tallest forest trees, of the 

 plants of the middle stratum, and of those residing on the ground 

 or on the lowest portions of tree trunks. It is particularly in 

 leaf structure that such difference is manifested; the trees pos- 

 sess relatively small, heavily cutinized leaves with deep palisade 

 tissue, frequently several layers in thickness, while the under- 

 trees and shrubs have leaves similar to those of temperate 

 deciduous trees, and the herbaceous plants have large thin leaves 

 with a single layer of chlorophyllous tissue and an extensive 

 development of intercellular spaces. These structural differ- 

 ences are chiefly correlated with the unlike water relation of the 

 foliage in the canopy and that on the ground, and with the dis- 

 similar light conditions. The rain-forest, in brief, makes its 

 own climate in so far as the plants of the lower strata are con- 

 cerned, and the steadying effect which the forest cover has upon 

 the moisture conditions within the forest and in the soil itself 

 is also of vital importance to the dominant trees. 



Even in the heaviest bodies of forest which clothe the desert 

 mountain ranges of Arizona there is no difference between the 

 physical conditions of floor and canopy except it be of very 

 infrequent and transitory occurrence. The long rainless periods 

 of the desert are common to the mountain tops as well, although 

 their desiccating effect is lessened by a shorter growing season, 



^ Shreve, Forrest, Studies on Jamaican Hymenophyllaceae. Bot. Gaz. 51: 

 184-209, 1911. 



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