258 LEAVES 



Lettuce, and some other plants grow better in some localities 

 under the shade afforded by slats or light cotton cloth. In green- 

 houses during the summer months it is usually necessary to 

 protect the plants against the intense rays of the sun either by 

 painting the glass or by some other means. Of course in shading 

 plants not only more favorable light for photosynthesis is often 

 provided, but the plants are also benefitted by being protected 

 from intense heat, excessive evaporation, and from hail and winds. 

 In many plants, as those of the Grass family, which seem to thrive 

 well under the direct rays of the sun, the surfaces of the leaves 

 slant so as to shun the intensity of the direct rays. 



On thd other hand, it is very common for leaves to be so 

 situated that they do not receive enough light. This is commonly 

 true of the lower leaves of the small grains, Clover, Alfalfa, and 

 other plants grown in thick stands. Often the leaves on the 

 interior branches of trees do not receive sufficient light. It is 

 for this reason that fruit trees with open heads have better light 

 relations for their interior branches than is afforded by trees with 

 a compact head. 



Plants growing in the house are usually insufficiently lighted, 

 especially if they are not very near a window. The problem of 

 overcoming so far as possible the insufficient lighting in green- 

 houses during the winter months is of primary importance in the 

 construction of greenhouses, being the deciding factor in the 

 selection of frame, and shape, quality, and thickness of glass. 



What should be considered active photosynthesis, as deter- 

 mined by the amount of starch produced per unit of time, varies 

 widely with different plants. However, investigations show that 

 a number of plants can produce 1 gram of starch per square 

 meter of leaf surface per hour under conditions favorable to active 

 photosynthesis. At this rate a leaf area of a square meter can 

 produce 10 grams of starch in a day of 10 hours. To do this, all 

 of the carbon dioxide would be taken from 250 cubic meters of 

 air. Carrying the calculation further in regard to the use of 

 carbon dioxide, it has been estimated that a yield of 300 bushels 

 of potatoes on an acre involves, including tops and all, about 

 5400 pounds of dry substance, and to form this, all of the carbon 

 dioxide over this acre to a height of 1^ miles would be used, 

 provided no carbon dioxide were added to the air in the mean- 

 time. This estimate emphasizes the importance of respiration, 



