198 AGRICULTURAL EXPERIMENT STATION. [Jan. 



and recombined into such organic comi")ounds ; though these 

 changes can take place only when the chlorophyll is 

 exposed to light of sufficient intensity, and when the water 

 supplied to it holds in solution suitable inorganic compounds 

 containing nitrogen, potassium, phosphorus, calcium, mag- 

 nesium, iron and sulphur. Under natural conditions, waters 

 from any soil in which plants will grow will be found to 

 contain all these substances ; but, in consequence of repeated 

 cultivation and removal of the crops, the supply of these 

 materials in a soil becomes greatly reduced, or, as we say, 

 the soil becomes "exhausted." It then becomes necessary to 

 supply the lacking constituents to the soil in the form of 

 manures or fertilizers ; and it is these necessary elements 

 which are commonly spoken of as "plant food." Being 

 inorganic, they cannot serve as food to the plant ; but, as we 

 have seen, their presence is indispensable to the elaboration 

 of the true food of the plant from the materials furnished by 

 water and carbon-dioxide. The precise relation of most of 

 these elements to the life of the plant is hardly at all under- 

 stood ; but it is easy to show that, in the absence of either 

 of them, there can be no permanently healthy activity. 

 Their relation to the elaboration of organic food material 

 from inorganic compounds has been compared, perhaps 

 aptly, to that of oil to the smooth running of a steam-engine. 



The necessary conditions being fulfilled, then, there 

 occurs a recombination of the constitutents of water and 

 carbon-dioxide into organic substance, excepting a part of 

 the oxygen, which is set free into the atmosphere. What- 

 ever temporary combinations they may pass through, the 

 first visible and stable form in which these recombined 

 elements appear is usually that of starch, which is the com- 

 monest form of organic food material that occurs in plants. 

 After it is thus provided, by the activity of* its chlorophyll, 

 with an organized food supply, the plant utilizes it, as it 

 needs, for the formation of tissue, either in repairing waste 

 or in new growth. 



But not all plants contain chlorophyll. Very many 

 resemble animals in being entirely unable to provide their 

 own nourishment, and in being, therefore, wholly dependent 

 on external sources of food supply. Since their food supply 



