78 BOTANY: PRINCIPLES AND PROBLEMS 



the excess, as heat, would sometimes raise the temperature of the 

 tissues dangerously, were it not absorbed in evaporating water 

 from the mesophyll cells. 



Transpiration is carried on primarily in the leaves, but may 

 occur in any other organs which are exposed to the air. Exces- 

 sive loss of water is often prevented in such regions by the 

 development of cell layers with corky walls. 



QUESTIONS FOR THOUGHT AND DISCUSSION 



200. Are leaves generally more variable in their area or in their 

 thickness? Explain. 



201. What general difference in shape is there between netted-veined 

 and parallel- veined leaves? 



202. Leaves on the same plant often differ markedly in the length 

 of their petioles. How do you explain these differences? 



203. Name three functions which the veins of a leaf perform. 



204. In leaves where the veins are much stouter than the thickness 

 of the blade, they usually stand out on the under side of the leaf and 

 thus leave the upper surface smooth. What are the advantages of this 

 arrangement to the plant? 



205. Of what advantage, and of what disadvantage, is it to plants 

 of temperate climates to shed their leaves in the winter? 



206. The leaves of "evergreen" trees do not remain permanently on 

 the tree but each year's crop lives only a few seasons and then drops off. 

 Why should these leaves not continue to live and function indefinitely? 



207. The upper epidermis and its cuticle are almost always thicker 

 than the lower. Explain. 



208. The cells of the leaf-epidermis usually have transparent walls 

 and a colorless cell-sap. Of what advantage is this to the plant? 



209. Some parasitic fungi attack only the epidermis of the leaf, but 

 they often cause the death of the leaf and even of the entire plant. 

 Explain. 



210. How do you explain the fact that the palisade layer is next the 

 upper surface of the leaf and the spongy layer next the lower? 



211. What does the plant gain by having the cells of the palisade 

 layer elongated at right angles to the leaf surface? 



