62 . THE HABITAT 
ple standard. The difference between the two values thus obtained repre- 
sents the amount of reflected light together with that screened by the epi- 
dermis. The amount of light transmitted through the leaf may be measured 
in the same way by using the leaf itself in place of the epidermis alone. 
The time of exposure is necessarily long, however, and it has been found 
practicable to obtain leaf prints by exposing the leaf in a printing frame, 
upon solio paper, at the same time that the epidermis,print is made. In a 
few species both the upper and lower epidermis can be removed and the 
amount of light absorbed determined directly by exposing the strip covered 
with the chlorenchym. Generally, 
however, this must be computed by 
subtracting the sum of the per cents 
of reflected and transmitted: light 
from 100 per cent, which represents 
the total light. 
90. Leaf and epidermis prints. In 
diphotic leaves the screening effect 
of the lower epidermis may be 
ignored. Isophotic sun leaves, i. e., 
those nearly upright in position or 
found above light-colored, reflecting 
soils, are usually strongly illumi- 
nated on both sides, and the ab- 
sorbed light can be obtained only by 
“measuring the screening effect of 
both epiderms. Shade leaves and 
submerged leaves often contain 
: chloroplasts in the epidermis, and the 
Fig. 15. Leaf print: exposure as before. above method can not be applied to 
Sumandshadeleavesol hilenlanulese) them, In fact, in habitats where the 
nella, Galium boreale, and Fotentilla light is quite diffuse, practically all 
bropingua. incident light is absorbed. The rare 
exceptions are those shade leaves with a distinct bloom. In addition to 
their use in obtaining the amount of light absorbed, both leaf and epidermis 
prints are extremely interesting for the direct comparison of light relations 
in the leaves of species belonging to different habitats. The relative screen- 
ing value of the upper and lower epidermis, or of the corresponding epiderms 
of two ecads or two species, is readily ascertained by exposing the two side 
by side in sunshine, over the slit in the photometer. For leaf prints fresh 
leaves are desirable, though nearly the same results can be obtained from 
