442 



MANUAL OF BOTANY 



The construction of organic substance proceeds in the cells 

 of the mesophyll containing the chloroplastids and supplied with 

 the raw materials in the way described in the last chapter. It 

 can, however, take place only under the influence of light, and 

 while the cells are at an adequate temperature. The complex pro- 

 ducts of the anabolic process are at first mainly, if not entirely, 

 confined to two classes, carbohydrates, and some nitrogen- 

 containing compound, probably proteid. Many other forms of 

 organic substance appear in plants, such as fats or oils, gluco- 



B C D 



Fig. 1186. 



a 



JO 



\20 



30 



4-0 50\ 60 70 80 ; 90 /OO 



I I h I I I I 1 I II I I 



I E m 



w 



Fig. 1186. Absorption spectra of clilorophyll and xantliophyll. ("After Kraus.) 

 The upper spectrum is given by an alcoholic extract of leaves, the middle 

 one by dissolving chloropliyll in benzol, and the lowest by xanthophyll. 

 The bands in the least refrangible portion, b-e, are figured as obtained 

 with a concentrated solution ; those in the most refrangible pai-t of the 

 spectrum, p-g, are given as obtained with a weak solution. The letters 

 B-G indicate the principal rays, tlie lumibers i-vii the absorption bands 

 of chlorophyll from red to violet, and the figures 0-100 divide the length 

 of the spectnim into 100 equal parts. (After Sachs.) 



sides, &c., but these are probably all produced by processes of 

 catabolism, and are not directly constructed from the simple 

 bodies absorbed. 



We will examine first the process of formation of carbohy- 

 drates. These consist of the elements carbon, hydrogen, and 

 oxygen, the latter two existing in the combination in the same 

 proportion as they do in water. The source of the carbon and the 

 oxygen is the CO., absorbed from the air, while the hydrogen is 

 derived from the water of the cell -sap. 



