256 THE MAINTENANCE OF THE INDIVIDUAL 



The Role of Water 



Water as a raw material needs little mention. The soil always con- 

 tains more or less water, and the original source of water in its cycle 

 through the oceans, the air, the clouds, and rain gives the earth a 

 never ending water supply. When mm aids Nature in carrying 

 water to dry areas by irrigation the desert literally is made "to blos- 

 som as the rose." Certain chemical elements find their way into the 

 plant body with this water. If the green plant is to manufacture 

 organic food substances, it is evident that the elements carbon, oxy- 

 gen, and hydrogen must come from the water and air. Various 

 mineral salts, taken in by the root, furnish the necessary amounts of 

 calcium, iron, potassium, sodium, and other elements, which leaves 

 only nitrogen to be accounted for. Although nitrogen makes up 

 approximately four fifths of the atmosphere, it is nevertheless unusable 

 in that free form. It is an extremely inert gas and does not unite 

 readily in combination with other substances. By means of the proc- 

 ess of decay, however, and particularly through the nitrogen-fixing 

 bacteria found on the roots of certain types of plants, this highly im- 

 portant element is made available to plants. So much for the raw 

 materials. Now let us turn to the machinery of food manufacture. 



Chlorophyll and Light 



Common observation shows that there is a relation between light 

 and the green color of plants. We are familiar with the bleaching of 

 celery stalks, with the curious blanched elongated shoots of a potato 

 which sprouts in darkness, and with the fact that young seedlings are 

 devoid of chlorophyll until after they have sprouted. Seedlings 

 grown with light coming from one side turn to the source of light, while 

 plants grown in a dark box having a hole on one side work their way 

 toward the light. Obviously light has a very potent effect on the 

 plant. 



Sunlight passed through a prism is broken up into seven primary 

 colors ranging from violet to red, but passed through a spectroscope 

 shows numerous dark lines traversing different areas in the spectro- 

 prism. The most conspicuous are used as landmarks by physicists 

 and for convenience have been designated by the letters A to H by 

 Fraunhofer, their discoverer. These several wave lengths of light can 

 be measured and it has been fovmd that they vary from 0.00076 mm. 

 at the red end of the spectrum to 0.00039 mm. at the violet end. 



