THAI'. HI] LIGHT 6i 



he trunks of trees and on rocky cliffs receive light from the front. Light 

 'rom behind or light from bclotv is usually of only slight importance, though 



noticed in Venezuela that a small species of Oncidium was always 

 ttachcd to the lower side of <he horizontal branches of the calabash-tree 

 Crescentia Cujcte). 



Of the two forms of daylight, direct sunlight is of less importance in 

 elation to plant-life than is diffuse light. M©sfe-pk-ftfes-©rth€f-expose only 



small part of their external su rface to the sun's rays or noncat all, but 

 eyond that they strive to avoid jhe sun's_i :a-VS by appropriate arrangements 

 nd movements of their foliage. 



The weakening of the intensity of light by branches and foliage is much 

 nore considerable than one would at first imagine. Wiesner', on a sunny 

 Jarch day (37th) in Vienna, estimated the intensity of the full daylight 

 t 0712, that at one hundred paces from the edge of the still leafless forest 

 ,t 0-355, and that under the shade of the trees at o-i66. 



The weakening of light under trees in full foliage, and especially in woods 

 n that condition, is naturally much more considerable than under bare 

 )ranches. Wiesner estimated the intensity of light in Vienna on a sunny 

 lay in March at c-666, but under the shade of a spruce-tree eight meters 

 ligh and branched nearly down to the ground, it was only o-02i ; on the 

 ame day, when the intensity of full daylight was 0-518, the intensity of light 

 aider a box-shrub, one meter high, was 0-017. At the beginning of May, 

 he intensities of the full daylight, of the light in the crown of a horse- 

 hestnut tree, and of the light under its shade, were respectively 0-500, 0-070, 

 nd 0-017. These values are in the ratio of 29 : 4 : i. 



How greatly the conformation of plants of sunny or shady habitats is 

 overned by light has been recently proved by comparative cultures 

 ndertaken by Wiesner. Sempervivum tectorum, for instance, is a typical 

 un-plant. With light of mean maximum intensity of 0-04, which is 

 ormal for many shade-plants, it abandoned its characteristic rosette-form, 

 ts internodes became elongated, its leaves diminished in size, and its 

 hlorophyll became reduced in quantity. The optimum for the growth in 

 he surface of its leaves in this case therefore coincides with a pretty high 

 ntensity of light ; at still higher intensities of light the superficial growth 

 s decreased. Wiesner cultivated plants of Sempervivum tectorum in some 

 ases with a mean light intensity of 0-305, in others with one of 0-152. 

 Jnder the first conditions the average maximum length of the leaves was 



1 mm. and their breadth 15 mm., whilst the corresponding figures under 

 he second conditions were 26 mm. and 13-5 mm. respectively. 



In other sun-plants, such as the potato and the bean, Wiesner observed 

 hat the growth of leaves was favoured by light up to a fairly high 



' Wiesner, III, p. 307. 



