16 BOTANY 



the green plant is the only organism which has the 

 power of forming organic substance from the inorganic 

 material of the earth and air. As all living beings are 

 dependent on this organic substance for the maintenance 

 of life, we see how the continuation of life itself upon 

 the earth depends on the activity of the green plant. 



The establishment of the position of the plant and its 

 defence when so established may be seen, therefore, to be 

 subordinate to the manufacture of organic food. 



The food so made is complex in character and will be 

 dealt with in greater detail in a subsequent chapter. 

 It comprises chiefly three classes of substance: carbo- 

 hydrates, of which sugar and starch are representatives, 

 fats, and proteins, which are much more complex in 

 composition, and are represented by the white of egg 

 and by the chief constituent of meat and fish. The 

 proteins are held to be the organic material which 

 most resembles the living substance itself. 



As it is the process of growth at the expense of this 

 newly constructed food, or of a small supply derived 

 directly from its parent, by 

 which the young plant makes 

 its way into its appropriate 

 position, it is clear that this is 

 the action of a living organism 

 and becomes probable that the 

 surroundings of the plant affect 

 FIG. 2. Geotropic curvature j 1 in other ways than by afford- 

 in root and shoot of ing it the material from which 



(After r Gibson.f Ufal ^ to make its food - Careful ob- 

 servation shows that this is the 



case. The root of the plant at even its first appear- 

 ance grows downwards in the direction of the soil. 

 If it be made to point in another direction, its plan 

 of growth slowly changes and it gradually curves 

 till its tip is pointing downwards again (Fig. 2). If 



