THE LEAF AND ITS FUNCTIONS . 79 



212. The upper surface of each epideriiial cell of the leaf Is often 

 slightly convex. Hoav may this perhaps be of advantage to the plant? 



213. Do you think that photosynthesis is carried on in the spongy 

 layer at all? What evidence have you for your belief? 



214. Name at least two advantages which the plant derives from 

 having its stomata more abundant on the lower surface of the leaf than 

 on the upper. 



215. Stomata are usually absent in the leaves of submersed water- 

 plants. Explain. 



216. Why does washing the leaves of house plants often improve the 

 health of the plants? 



217. In cities where soft coal is burned and there is much coal-dust in 

 the air, all trees suffer, and evergreen trees in particular find it very 

 difficult to hve. Explain these facts. 



218. In what ways is the supply of carbon dioxide in the atmosphere 

 being replenished ? 



219. Do you think that the continual removal of carbon dioxide from 

 the atmosphere by green plants in photosynthesis will reduce the 

 amount of this gas which is present there? Explain. 



220. The great deposits of coal formed in the Coal Period, millions 

 of years ago, have sometimes been pointed to as evidence that carbon 

 dioxide was much more abundant in the atmosphere then than now. 

 What basis is there for such a conclusion? 



221. As the carbon dioxide in the leaf is removed by photosynthesis, 

 what causes a fresh supply to enter the stomata from the outside air? 



222. What factors determine the rate at which carbon dioxide enters 

 the leaf? 



223. In some greenhouses the plants are "fertilized" by pouring 

 carbon dioxide from a chimney into the air around the plants. Why 

 does this increase plant growth and why would it be impracticable 

 out-of-doors? 



224. The fact that carbon dioxide is more soluble in cold water than 

 in warm has been cited as one of the reasons why seals, walruses, polar 

 bears, and even Eskimos are able to thrive in such large numbers so 

 far north. Explain. 



225. A solution of chlorophyll outside the plant, when exposed to 

 sunlight and in the presence of carbon dioxide, will not produce sugar. 

 How do you explain this? 



