Value oS Light 



of light upon the leaf reduces the power at the 

 disposal of the leaf ; even glass of ordinary kinds 

 has that effect. 



Curiously enough, very bright Hght has a destruc- 

 tive effect upon the green colour, but the plant has 

 the power of protecting it from destruction by 

 removing the grains containing it from the direct 

 rays of the sun when these threaten it too severely. 

 Plants which naturally grow in more or less deep 

 shade are able to make use of even the faint Ught 

 that reaches them, but it may be so faint that 

 although they can make enough food by its aid 

 to sustain existence, yet they make insufficient to 

 enable them to flower. 



The making of sugar may be looked upon as the 

 beginning of the food-making process, for plants, 

 no less than animals, require other foods. The next 

 step in building the more complex and valuable 

 foods is the addition of nitrogen to the carbon, 

 hydrogen, and oxygen of the sugar. The nitrogen 

 is derived from the soluble nitrates and salts of 

 ammonia absorbed with the soil-water. After this 

 combination is effected, sulphur from sulphates and 

 phosphorus from phosphates, both of which have 

 similarly come from the soil, must be brought into 

 chemical combination with the other elements before 

 the material is available out of which the Hving stuff 

 itself is made. The steps by which these combina- 

 tions are brought about, and the conditions upon 



13 



