FACTORS INFLUENCING PHOTOSYNTHESIS 257 
combining the elements of sugar, which is the foundational sub- 
stance, with nitrogen, sulphur, and phosphorus derived from the 
mineral salts of the soil. Even if the construction of proteins in 
leaves does not depend upon light, it is obvious that leaves are 
well equipped for such 
work, since they manufac- 
ture sugar and the water 
brought up from the soil 
supplies them with an 
abundance of mineral salts. 
Factors Influencing 
Photosynthesis. — The 
factors influencing photo- 
synthesis are light, temper- 
ature, moisture, and 
amount of chlorophyll. 
That light is absolutely 
essential for photosynthesis 
is easily demonstrated by 
applying the iodine tests to 
two sets of leaves after one 
set has been kept in the 
dark and the other in the 
light for a few days. Even 
by shading only a portion 
of a leaf the necessity of 
light for photosynthesis can 
be demonstrated as shown 
in Figure 235. For photo- 
synthesis sunlight is best, 
although some photosyn- 
thesis will take place in 
artificial light that ‘has a 
suitable intensity. It has 
been demonstrated in 
Fic. 235. — A leaf showing the relation 
of photosynthesis to light, as indicated by 
the amount of starch formed. After cover- 
ing the area represented by the light band, 
the leaf was left exposed to the sunlight for 
a few hours, then removed from the plant 
and the iodine test applied. The area pro- 
tected has no starch while the areas exposed 
‘are quite dark, due to the presence of much 
starch. Adapted from Palladin. 
greenhouses that some plants, at least, carry on photosynthesis at 
night if the proper kind of electric light is provided. For many 
plants the direct rays of the sun are too intense, in which case 
photosynthesis is most active in strong diffuse light. It is 
partly for this reason that Pineapples, Tobacco, Potatoes, Cotton, 
