ing space also takes place by osmosis through the cell walls. If we think of 

 the ingoing and outgoing gases, and disregard the chemical changes in 

 which the gases take part, we may speak of this process as respiration, or 

 breathing. Stomata in the epidermis, or skin, of young twigs connect with 

 the intercellular spaces below the surface (see illustration, p. 142). In the 

 older twigs, however, in which bark-formation has been going on for some 

 time, the live cells beneath the bark get their oxygen supply by way of the 

 lenticels. The comparatively small amounts of oxygen used by the plant 

 cells diffuse slowly into them from air in these openings and passages. The 

 carbon dioxide from the cells diffuses to the exterior along the same paths. 



In most plants the stomata, or breathing holes, are located on the under 

 side of the leaf. In water-lily pads and similar floating leaves, these openings 

 are on the upper surface, where they are exposed to the air. In some plant 

 species, variation in leaf structure seems definitely related to respiration. 

 Leaves exposed to air "breathe'' through stomata, whereas submerged leaves 

 carry on gas exchange by osmosis through the general surface. 



Respiration in Roots The roots of most familiar plants and staple 

 crops, with the exception of rice, absorb oxygen dissolved in the moisture 

 on the outer surfaces, and also give out carbon dioxide by osmosis. Most 

 roots suffocate when the water table is too high — that is, when the free 



INCOMES AND OUTGOES OF A LIVING CELL 



In the body of one of the larger or more complex animals, each cell receives oxygen, 

 as well as food, by diffusion from the surrounding fluid. Each cell discharges into 

 this surrounding fluid carbon dioxide, as well as urea and other products of metabo- 

 lism — also by diffusion through the cell wall. The fluid, or lymph, communicates in 

 turn with the blood stream 



202 



