Chap. I] CHARACTERS OF THE TROPICAL CLIMATE 223 



II. Gratz. 



ii. PHYSIOLOGICAL ACTION OF TROPICAL LIGHT ON PLANTS. 



From our knowledge of the effects of light on growth and transpiration, 

 we may safely assume that the light of the tropics, in accordance with its 

 greater intensity, influences these processes still more strongly than does 

 the weaker light of higher latitudes. In fact the growth in length of a 

 bamboo is quite strikingly less during the hours of the day than during those 

 of the night ; the growth in length of the aerial roots of Taeniophyllum 

 Zollingeri stops altogether in daylight of relatively very moderate intensity, 

 even in a very weak light ; and the comparatively strong transpiration, 

 which causes the flaccid drooping of the foliage of many tropical plants 

 during the brightest hours of daylight, may be considered partly as an 

 effect, even if indirect, of the rays of light. 



The share that light takes in the above-mentioned functions has not 

 yet been numerically determined. However a glimpse into the different 

 specific actions of light has been afforded, in particular by Wiesner's 

 observations at Buitenzorg. 



The lie in relation to light of the leaves of tropical plants, according 

 to Wiesner's observations, differs in general from that of the leaves of 

 temperate plants, since they do not, like them, universally strive to attain 

 a position perpendicular to the strongest diffuse light, but exhibit this 

 disposition only in the inner, poorly lighted part of the crown ; at the 

 periphery of the tree, however, they give up this disposition in the face 

 of rays of high incidence, and the lie of the leaves is there determined 

 by direct sunlight. 



Owing to this lie of the leaves when they are exposed to the direct 

 rays of the sun, not only are strong heating and consequent excessive 

 transpiration prevented, but at the same time the destruction of the 

 chlorophyll by intense light is obviated. Several characteristics of tropical 



