ADJUSTMENT TO LIGHT 77 



3 altogether too small to have any important effect. The amount 

 f light received by a slope differs from that received by a level 

 xea of the same extent. Since the leaf position is the same in 

 toth cases, this difference is of no significance. 



Experiment 21. Measuring light intensity. Make a standard be- 

 ween 10 a.m. and 2 p.m. on a sunny day, and then take a reading in 

 he open, one in the plant-house, and one in each shade-tent. Repeat 

 he series at the same time upon a cloudy day. Remove the strip, 

 ,nd find the value of each reading. Is the ratio of intensity in the 

 arious places the same for cloudy and for sunny days? 



96. Reception and absorption of light. Plants possess no 

 pecial structures for the reception of light. The latter falls alike 

 ipon all aerial parts, the characteristic form of the leaf being 

 hiefly to increase the surface. The epidermal cells, which re- 

 eive the light, merely transmit it, and the stimulus is first felt 

 a the chlorenchym beneath. The general effect of the epitlermis 

 5 to reduce the amount of light that enters the leaf. This is due 

 lartly to reflection and partly to absorption. Leaves with a 

 mooth or shining cuticle reflect the light, while a thick cuticle 

 r a dense coating of hairs absorbs much of it. In some cases 

 he shape of the epidermal cells is such that the light passing 

 hrough the cuticle is more or less concentrated before it reaches 

 he chlorenchym. This is a rare occurrence, however, and of 

 ttle importance. The rule is that by far the greater amount 

 f the light that enters the leaf passes through the epidermis and 

 ) absorbed by the chlorenchym. Some light passes entirely 

 tirough the leaf, but ordinarily this is slight. Thick or flesh}' 

 ;aves absorb practically all the light that falls upon them. Thin 

 iaves placed in sunshine transmit considerable light, but it must 

 e remembered that such leaves are usually confined to shady 

 abitats, where the light is very diffuse and the absorption rela- 

 .vely complete. 



97. The amount absorbed. The amount of light actually used 

 y the leaf is ascertained by determining the amount that passes 

 irough the epidermis, and by taking from this that which passes 

 irough the entire leaf. The amount of light available for the 

 hlorenchym is measured by stripping a piece of epidermis from 

 le leaf. This is placed over the opening of the ])hotometer, and 

 n exposure made. After the strij) is removed, another exposure 



