LIGHT : 61 
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, 1; 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 approximation 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 light 
transmitted through the epi- 
dermis. This is done by 
stripping a piece of epidermis Fig. 14. Leaf print: exposed 10 m., 11 a.m. Au- 
ust 20. The leaves are from sun and shade 
from the upper surface of the orms of - Bursa bursa-pastoris, Rosa sayii, 
( i i Thalictrum sparsiflorum, and Machaeranthera 
rei a piseiig ur ihe aspera. In each the shade leaf prints more 
slit in the photometer for an deeply. 
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 kalf of the opening, leaving the other half to record the 
full light value, and the results, or epidermis prints, are referred to a multi- 
