THE LEAF AND ITS FUNCTIONS 81 



242. Seedlings of most forest trees will grow and thrive on the forest 

 floor in rather deep shade, but the lower limbs of mature trees of the 

 same species, if subjected to similar light conditions, will soon die. Of 

 what advantage to the plant is this difference between young and old 

 individuals? 



243. Most of our woodland "wild flowers" (herbaceous plants which 

 grow on the forest floor), thrive and blossom in early spring. Why? 



244. Where does the light come from which is utilized by the chloro- 

 phyll of the spongy layer? 



245. Leaves exposed to bright sunlight are thicker than those grow- 

 ing in the shade, even on the same plant. Explain. 



246. How would you determine what wave-lengths of light are of 

 most importance in photosynthesis? 



247. Would you predict that a plant would thrive better in red light 

 or in green light? Why? 



248. Plants grown in winter in greenhouses rarely grow as rapidly 

 as do plants out-of-doors in the summer, even though the temperature 

 is kept as high. Explain. 



249. Other things being equal, why do most plants grow faster in 

 June than later in the summer? 



250. Crops are necessarily planted much later near the northern limit 

 of their range than farther south, but in many cases their growth-rate 

 is so much more rapid that they reach maturity almost as soon. Explain. 



251. In the North Sea fisheries, it has been found that the size of the 

 season's catch of fish tends to be greater in seasons when there has been 

 much sunshine than in seasons which have been relatively cloudy. 

 Explain. 



252. Some plants (such as the fungi) are able to thrive in the absence 

 of light. What other important phj'siological differences must there 

 be between these and ordinary green plants? 



253. Of what advantage is it to the plant to have the sugar which is 

 produced by photosynthesis converted rapidly into starch? 



254. It is sometimes said that forests tend to "])urify the air." 

 What basis in fact is there for this statement? 



255. Why do animals in an acjuarium thrive l)ctter if green water- 

 plants are growing in the aquarium? 



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