Ch. Ill, 3] SYNTHESIS OF FOOD 27 



source of power, and the chlurophyll the mo.chinery by 

 which it is apphed to the work : carbon dioxide and water 

 are the ra v: ma terials, sugar the desired manufactured prod- 

 uct, and oxygen an incidental hy-product. The comparison 

 while fanciful in details, is correct in essentials. 



Grape sugar is, however, not the only food material formed 

 in the leaves, for they are also the places of construction of 

 PROTEINS. These are substances of the greatest importance 

 in plant life, because they constitute the foundational ma- 

 terial of the lining protoplasm. They are composed of the 

 elements of the grape sugar, — carbon, hydrogen, and 

 oxygen, — together T\1th nitrogen, sulphur, and phosphorus 

 derived from mineral compounds absorbed from the soil and 

 brought to the leaves T^-ith the water. Proteins, though 

 many and diverse, are all constructed from grape sugar 

 by chemical addition of the other constituents, — nitrogen 

 first, and the others later. Unfortunately we know little 

 as j'et, despite many researches, as to their exact place of 

 formation in the leaves, whether in the veins or the 

 chlorenchjina. They occur abundantly in the veins, along 

 which they are conducted into the stem. Xor is it certain 

 whether light is essential to their formation, though the 

 e\'idence seems to show not, in which case the energj- needed 

 in their s}-nthesis must be suppHed bj' chemical action. 

 Probably their formation in the leaves is only a functional 

 convenience based on the .simultaneous presence there of 

 the basal grape sugar and the needful mineral matters, 

 brought with the water. These proteins, hke the grape sugar, 

 move continuouslj^ along the veins from the leaves to the 

 stems. 



The role of the grape sugar thus formed in leaves is very 

 fundamental in plant hfe. First, from it, or from the pro- 

 teins built upon it, plants build, by minor chemical trans- 

 formations, their entire structure, and form all of the many 

 organic materials in their bodies, as will later appear in detail 

 in a separate section. Second, the energy of the simlight, 



