RESPIRATION IOQ 
in carbon dioxide than the air outside, and therefore this 
gas passes out, also by diffusion. 
111. Lenticels. Many living cells and tissues are more 
deep seated than the mesophyll of leaves, and oxygen 
obtains access to these cells by different ways in different 
plants. Only one of these cases may be considered here. 
If any young woody twig is examined, small "dots" or 
lines will be discovered on the surface. On closer exami- 
nation these will be found to be small openings through 
the bark (Fig. 70). They are known as lenticels. The 
outer portion of the bark, and the older, inner layers 
of wood are not alive, but the cambium layer, between 
wood and bark is alive, and therefore needs a continual 
supply of fresh oxygen. This is supplied through the 
lenticels in a manner quite analogous to that by which 
the supply in the leaves is maintained. 
112. Respiration and Photosynthesis. The two proc- 
esses of respiration and photosynthesis are often con- 
fused, but in reality thay have very little in common, 
except that both result in an exchange of gases with 
the external air. But it must be kept clearly in mind 
that the processes themselves are quite distinct from 
the exchanges of gases that accompany them, or result 
because of them. Photosynthesis furnishes carbon to the 
plant in a form available for use; respiration is the 
physiological process by which the carbon is utilized to 
supply the energy necessary for all life-processes. The 
result of the two processes is the continual income and 
outgo of carbon. The carbon enters and leaves the 
plant in the same form, namely carbon dioxide. The 
disintegration of carbohydrates is also accomplished by 
bacterial decay and other fermentative processes. We 
