of carbon dioxide for photosynthesis, 
THE LEAF D9 
necessary for the entrance of carbon dioxide into the cells. Since 
the wet surfaces of the cells are in contact with the air in the 
intercellular spaces, some of the water in the walls vaporizes and 
diffuses into this air, just as the water from wet clothes goes 
into the surrounding atmosphere while the clothes are drying. 
The accumulation of water vapor in the air in the intercellular 
spaces makes the water content of this 
air greater than that of the atmosphere 
surrounding the leaf, so that water 
vapor diffuses out through the stomata 
into the external atmosphere. The dif- 
fusion of water vapor from the intercel- 
lular spaces into the atmosphere tends 
to reduce the water content of the air in 
the intercellular spaces, and thuscauses 
additional water to leave the cell walls 
and enter the intercellular spaces. The 
passage of water from the cell walls 
into the intercellular spaces, and then 
out through the stomata into the ex- 
ternal atmosphere, is thus continuous 
as long as the cell walls are wet and 
the stomata are open. Since those con- 
ditions are necessary for the absorption 







Fic.41. Apparatus for dem- 
onstrating the loss of water 
: by transpiration 
transpiration 1s a constant concomitant 
of photosynthesis. In other words, the conditions that are nec- 
essary for photosynthesis make transpiration unavoidable. Tyran- 
spiration is, strictly speaking, not a process of the plant itself but 
the result of the action of external conditions on the plant. 
Some water is lost from the epidermal cells. This is known 
as cuticular transpiration. The amount of water given off in this 
manner is, however, small as compared with that which diffuses 
out of the stomata. 
Demonstration. That water is lost by transpiration can be dem- 
onstrated by the arrangement shown in Fig. 41. A branch is 
