1890.] PUBLIC DOCUMENT — No. 33. 197 



by it into vegetable tissue. Such substances constitute the 

 real food of plants, in the same sense that what an animal 

 eats constitutes its food ; and both plants and animals find 

 available food-supply only in organic substances. Inorganic 

 materials can no more serve plants than they can serve 

 animals as food ; and just here a distinction must be made 

 between the true food of plants and " plant food," so called 

 in the discussion of fertilizers. We shall see later what is 

 the relation to the plant of the latter, which consists 

 essentially of inorganic substances. 



Now, we know that an animal must obtain its food 

 materials ready formed ; that is, it cannot prepare the 

 organic nutriment it requires from inorganic substances, but 

 must obtain it from plants or from other animals. Here lies 

 the important distinction between animals and green plants ; 

 for, in spite of the fact that, to most persons, the y^ovd. plant 

 carries with it the idea of greenness, it is by no means true 

 that all plants are green. Green plants owe their color to 

 the presence in their leaves and other green parts of a special 

 pigment, known as leaf-green or chlorophyll. It may be 

 added that some plants which are not green to the eye, yet 

 contain chlorophyll, whose presence is hidden by some other 

 masking pigment. The term " green plants" is here used, 

 then, to designate all chlorophyll-containing plants, what- 

 ever their external appearance. 



In chlorophyll we have the remarkable substance which 

 bridges the gap between the inorganic and the organic. It 

 is the one substance in nature on whose activity the continu- 

 ance of all life depends. It alone has the power of forming 

 organized food materials out of the elements of inorganic 

 substances, but only under certain definite conditions. The 

 green tissues of land plants receive water from the soil by 

 way of their roots and stems, and absorb from the atmos- 

 phere the carbonic acid gas, or carbon-dioxide, which it 

 contains in small proportion. These two simple inorganic 

 compounds, water and carbon-dioxide, furnish the elements, 

 carbon, hydrogen and oxygen, for the formation of certain 

 organic compounds ; and it is the peculiar property of 

 chlorophyll, that, in its presence, and in its presence only, 

 these elements are freed from their original combinations, 



