RESPIRATION IO9 



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 



