1904] BERGEN—TRANSPIRATION OF EVERGREENS 295 
These considerations, however, do not explain all of the observed 
inequalities of transpiration. Portions of leaves of Agave americana 
freshly cut and with the cut surfaces hermetically sealed with Wax, so 
as to permit no loss of water except through uninjured epidermis, were 
found to give ratios ranging from 1.5 to 4 for loss of sun leaves com- 
pared with that of shade leaves, when both were exposed to full 
sunlight. Here the transportation of water is an unimportant factor, 
and the amount of tissue inside the leaf from which the transpired 
water is drawn was nearly the same in both cases. The Agave 
shade leaves had grown in a permanent shade of about 2 per cent. 
illumination. 
: Fic. 11.—Lower epidermis and mesophyll of Pistacia: A, sun leaf; B, shade leaf. 
125. 
It may be of interest to append a statement of the absolute rate 
of transpiration of the four trees and shrubs discussed in the table 
above given, The measurements were made with sun leaves a year 
old, at a temperature of 21° C. and a relative humidity of 67 per cent. 
The leaves Were in moderately bright sunlight. | 
TRANSPIRATION IN MG. PER 10051 LEAF SURFACE PER HOUR.?° 
a “pparently large values for the transpiration of some- 
Sativym, = bus Plants. Leaves of Ulmus campestris and of Pisum 
“ade re examined at the same time, for purposes of comparison, 
_"€T€ found to lose 342 and 353™ of water per hour, respectively. 
however, only serves to emphasize a fact too often lost sight of, 
to Onl 
¥ one surface of each leaf (the lower) is taken into account. 
