30 



Science of Plant Life 



Fig. 19. A maple leaf and the sugar and maple 

 sirup equivalent to the amount it could manu- 

 facture in a season. All drawn to the same 

 scale. 



night the movement of food into the stem nearly empties 

 the leaf, and by early morning the cells are again in good 



condition for food manu- 

 facture. The waste prod- 

 uct, oxygen, passes from 

 the cells to the intercellu- 

 lar spaces and out through 

 the stomata to the atmos- 

 phere. 



A leaf, then, is carrying 

 on photosynthesis at its 

 full capacity only when 

 there is sunlight, a favor- 

 able temperature, and an 

 abundant water supply, 

 and when the stomata are open. Even under these condi- 

 tions the work may be interfered with if more than a certain 

 amount of the products accumulate in the cells. 



The amount of the product. The amount of carbohydrates 

 produced in photosynthesis varies so greatly in different plants 

 and under dissimilar conditions that it is very difficult to 

 make a general estimate of it. The result of many experi- 

 ments shows that under favorable conditions a square meter 

 of leaf surface makes on an average about i gram of car- 

 bohydrate per hour. At this rate a square meter of leaf sur- 

 face in midsummer would require 2 months to produce food 

 equivalent to that consumed by the average man in a day. 

 This average rate of carbohydrate manufacture may also be 

 expressed by saying that the leaf makes enough sugar 

 in a summer to cover it with a layer i mrllimeter thick 

 (Fig. 19). 



