Chap. Ill] LIGHT 61 



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

 from behind or light from below is usually of only slight importance, though 

 I noticed in Venezuela that a small species of Oncidium was always 

 attached to the lower side of the horizontal branches of the calabash-tree 

 (Crescentia Cujete). 



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

 relation to plant-life than is diffuse light. Most plants either expose only 

 a small part of their external surface to the sun's rays or none at all, but 

 beyond that they strive to avoid the sun's rays by appropriate arrangements 

 and movements of their foliage. 



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

 more considerable than one would at first imagine. Wiesner 1 , on a sunny 

 March day (27th) in Vienna, estimated the intensity of the full daylight 

 at 0-712, that at one hundred paces from the edge of the still leafless forest 

 at 0-355, and that under the shade of the trees at 0-166. 



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

 in that condition; is naturally much more considerable than under bare 

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

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

 high and branched nearly down to the ground, it was only 0-021 ; on the 

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

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

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

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

 and 0-017. These values are in the ratio of 29 : 4 : 1. 



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

 governed by light has been recently proved by comparative cultures 

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

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

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

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

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

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

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

 is decreased. Wiesner cultivated plants of Sempervivum tectorum in some 

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

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

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

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



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

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



1 Wiesner, III, p. 307. 



