38 



General Botany 



Fig. 22. 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. 



average man in a day. This average rate of carbohydrate manu- 

 facture may also be expressed by saying that the leaf makes 



enough sugar in a summer 

 to cover it with a layer i 

 millimeter thick. Because as 

 a whole the factors involved 

 in photosynthesis are most 

 favorable during the morning 

 hours, the greater part of 

 food manufacture occurs be- 

 fore noon. 



An acre of corn exposes 

 about 2 acres of leaf surface 

 to the light. The total 

 weight of organic material in an acre of mature corn plants 

 having a yield of loo bushels of corn is about 7 tons. Of 

 this amount, about 3 tons is carbon. To secure such a large 

 quantity of carbon, not less than 11 tons of carbon dioxide were 

 taken in by the plants. Furthermore, as we shall see in connec- 

 tion with respiration, not all the carbon taken in and built into 

 organic compounds remains in the mature plants. It is estimated 

 that the plants of the United States manufacture nearly a cubic 

 mile of sugar each year. 



Hindrances to photosynthesis. Aside from the lack of light, 

 water, and carbon dioxide, the process of photosynthesis may be 

 interfered with in several ways. In cities where there are much 

 dust and smoke, plants do not grow well because (i) the amount 

 of sunlight is greatly reduced ; (2) the dust forms a layer on 

 the upper surface of the leaf and reduces still further the amount 

 of light that actually reaches the chlorenchyma ; and (3) soot 

 and dust collect in the stomata and interfere with the entrance 

 of carbon dioxide. If the dust and smoke are very abundant, 

 the stomata may even become completely blocked and photo- 

 synthesis stopped altogether. 



