NUTRITION. 



167 



more complex substance. This energy cannot be supplied 

 by the plant itself. An external source of energy is therefore 

 necessary. What this source is is unimportant, provided the 



Fig. 173. — The absorption spectrum of an alcoholic solution of chlorophyll. A beam 

 of sunlight passed through a prism is broadened into a strip, called the spectrum, 

 which shows different colors, according to the length of the light waves, the longest 

 appearing red and the shortest violet. Some of the light waves are stopped by 

 absorption, and at these places black lines appear (Fraunhofer lines), the more im- 

 portant being those below the letters B, <-', etc. When the sunlight passes through an 



alcoholic solution it absorbs those parts of the light corresponding to the dark bands 



lade visible ' 



:he Fran 

 or roughly by the colors. — After Kraus. 



I to VII. These absorption bands are made visible by spreading out the light ray into 

 a spectrum. The bands are located by the Fraunhofer lines, or by the artificial scale, 



energy be sufficiently intense. The light of an electric arc 

 serves the purpose as well as sunlight, if its intensity be 

 equal. 



232. (c) Potassium salts. — These take no part in the 

 composition of the food produced, and their exact role is not 

 understood. It is well established, however, that their 

 presence is essential to the formation of the carbohydrate. 



233. The product of photosyntax. — The steps in the 

 process of the building of carbohydrates are not thoroughly 

 known. Present indications are that the material first pro- 

 duced by the rearrangement of the molecules of carbon, 

 hydrogen, and oxygen, derived from the carl ionic acid, is a 

 molecule of the simplest carbohydrate, formaldehyde. CI !..< >. 

 Several of these are then built up (by condensation and 

 polymerization) into one of the more complex carbohydrates, 

 such as cane sugar. Starch is generally the first visible prod- 

 uct and appears as minute granules in the interior of the 



