ILLUSTRATIVE STUDIES 135 



Thus the flow of gases by diffusion throughout the plant body 

 while in necessary obedience to physical laws is yet in direct 

 accordance with physiological demands. 



Movements of the atmosphere en masse throughout the 

 plant body are accomplished by differences between external 

 and internal pressures resulting from fluctuations of tempera- 

 ture within and without, and by compression and expansion 

 of the intercellular spaces caused by swaying of the plant to 

 and fro, and by fluctuations in the turgidity of the tissues as 

 the ratio between absorption and transpiration rises and falls. 

 Thus when the temperature of the plant body falls below that 

 of the exterior the air in the intercellular spaces contracts and 

 outside air flows in, and the reverse process takes place when 

 the temperature of the plant rises above that of the outside 

 atmosphere. Again, when transpiration is going on faster 

 than absorption by the roots the cells lose in turgidity, con- 

 tract, and leave more room for the intercellular spaces, and 

 pressure from without forces more air in. When transpira- 

 tion subsides following the overclouding or going down of the 

 sun, or increase in the atmospheric humidity, the cells become 

 more turgid and compress the intercellular spaces so that the 

 air in them is in part forced out. The relatively rapid exchanges 

 of air brought about in these ways may be of decided value to 

 the functions employing oxygen and carbon dioxide. Free nitro- 

 gen, not being employed by the higher plants, may here be left 

 out of the discussion of gaseous interchange. 



ILLUSTRATIVE STUDIES 



i. Study the stomata of different leaves. Bleach and clear 

 the leaves as described in Illustrative Studies, paragraph 7 of 

 the last chapter. Study with both low and high powers. With 

 objects so transparent as these it is best to illuminate with oblique 

 light by setting the mirror-bar at an angle with the perpendicular. 

 Mount the leaf or a portion of it in chloral hydrate, first with 

 the under side up. With the eyepiece micrometer, and using 



