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- 





Fig. 235. — A leaf showing the relation 

 of photosynthesis to Kght, as indicated by 

 the amount of starch formed. After cover- 

 ing the area represented by the light band, 

 synthesis sunlight is best, the leaf was left exposed to the sunUght for 

 although some photosyn- a few hours, then removed from the plant 

 thesis will take place in and the iodine test apphed. The area pro- 

 J.-C. ■ 1 1- 1-i xu J. I. tected has no starch while the areas exposed 



artificial light that has a i. j i j ^ ^.i, r , 



^ - . . ^ , are quite dark, due to the presence of much 



smtable intensity. It has gtarch. 



been demonstrated in 



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, 



