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 the 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 chief concern in greenhouse 
construction, determining largely the quality, thickness, and 
shape of the glass, and the nature of the frame. 
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 14 miles would be used, 
provided no carbon dioxide were added to the air in the mean- 
time. This estimate emphasizes the importance of respiration, 
