CHAPTER XIII. 



WORK OF LEAVES (Continued). 



II. PHOTOSYNTHESIS. 



165. The formation of sugar and starch during photo- 

 synthesis. The work done by leaves in the formation of 

 sugar and starch requires the air and light relations.* In the 

 case of aquatic plants the air is mixed in the water so that fun- 

 damentally, so far as the process is concerned, the relations of 

 land and aquatic plants to light and air are the same, though it is 

 better to speak of the light and water relation of aquatic plants. 

 The thin and broad form of the leaf exposes a large surface to 

 the air and light, while the stomates and intercellular spaces of 

 the leaf afford free communication of air into the mesophyll 

 portion of the leaf. That sugar and starch are plant products 

 we have learned from our study in Chapters II and III. Now 

 we wish to learn the process by which the plant makes them. 



166. Need of the light relation. The leaves of the corn 

 plant, the beet, onion, and most of the monocotyledons which 

 have been exposed to the light contain a certain amount of sugar. 

 On the other hand the leaves of the bean, tropaeolum, potato 

 and most of the dicotyledons which have been exposed to the 

 light contain starch, which can be demonstrated by dissolving 

 out the chlorophyll and treating the leaf with a tincture of iodine. 

 A small quantity of sugar is also present. Starch disappears 

 from leaves which have been kept in the dark during the night, 

 and if the leaf, or a portion of it be kept in the dark the following 

 day, or in very weak light, no starch will appear. This clearly 

 indicates that light plays an important part in this work of the 

 leaf and explains the need of the light relation. 



* In the case of aquatic plants the relations are light and water, the water 

 containing air mixed with it. 



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