LIGHT 



6l 



received, by means of readings with solio paper, and the knowledge thus 

 obtained is of great importance in interpreting the modifications of certain 

 types of leaves. For example, a leaf with a densely hairy epidermis may 

 receive light of the full intensity, i ; the amount reflected or screened out by 

 the hairs may be 95 per cent of this, the amount absorbed 5 per cent, and 

 that transmitted, nil. In the majority of cases, however, the absorbed light 

 is considerably more than the 

 amount reflected or trans- 

 mitted. 



89. Methods of determina- 

 tion. If results are to be of 

 value, reflected and transmit- 

 'ted light must be determined 

 in the habitat of the plant 

 simultaneously with the total 

 light which a leaf receives. 

 An approxiination of the 

 light reflected from a leaf 

 surface is secured by placing 

 the photometer so that the 

 light reflected is thrown upon 

 the solio strip. A much more 

 satisfactory method, however, 

 is to determine it in connec- 

 tion with the amount of Ught 

 transmitted through the epi- 

 dermis. This is done by 

 stripping a piece of epidermis 

 from the upper surface of the 

 leaf and placing it over the 

 slit in the photometer for an 

 exposure. An exposure in 



the full light of the habitat is made simultaneously with another photometer, 

 or immediately afterward upon the same strip. When the epidermis is 

 not too dense, both exposures are permitted to reach the same tint, and the 

 relation between them is precisely that of their lengths of exposure. Or- 

 dinarily the two exposures are made absolutely simultaneous by placing 

 the epidermis over half of the opening, leaving the other half to record the 

 full light value, and the results, or epidermis prints, are referred to a multi- 



Fig. 14. Leaf print: exposed 10 m., 11 a.m. Au- 

 gust 20. Tlie leaves are from sun and shade 

 forms of Bursa bursa-pastoris, Rosa- sayii, 

 Thdlidrum sparsiflorum, and Machaeranthera 

 aspera. In each the shade leaf prints more 

 deeply. 



