228 PLANT PHYSIOLOGY 



These laws, which the movement of gases obeys, should be 

 kept well in mind, because we shall see in the next chapter that 

 the movements of liquids and solids are subject to the same laws. 



QUESTIONS 



1. Why are leaves less variable in thickness than in area? 



2. Why is the palisade tissue on the upper side of the leaf and the spongy 

 tissue on the lower? 



3. Why does washing the leaves of plants generally improve their con- 

 dition? 



4. Which suffer most from coal dust in the air, — evergreens or deciduous 

 trees? Why? 



5. Why does the addition of carbon dioxide to the air benefit the plant? 



6. Is carbon dioxide more soluble in warm or cold water? What effect has 

 this upon life in the arctic regions? 



7. Would it be of any advantage if the stomata were more numerous? 



8. If ordinary air were separated from a mixture containing half carbon 

 dioxide and half oxygen by a water membrane, which gas would diffuse more 

 rapidly into the air? Why? 



9. Dangeard reported that he had kept leaves of Aucuba in a vacuum for 

 6 months. How is this explained? Were the leaves kept in the light or dark? 



REFERENCES 



Andrews, F. M. — Quantitative estimation of aeration in leaves. Proc. Ind. 



Acad. Sri. ,38:268, 1922. 

 Hawthorne, W. C. — Diffusion, osmosis, and osmotic pressure. Sri. Mo., 



31:534, 1930. 

 Zimmerman, P. W., Hitchcock, A. E., and Crocker, Wm. — The movement 



of gases into and through plants. Contr. Boyce Thompson Inst., 3:313, 



1931. 



