44 Plant Life and Evolution 



eliminated by revolving the plant so that all por- 

 tions are equally exposed to the influence of gravity 

 this curvature will be absent. The many mechanical 

 stresses due to the action of gravity must exercise 

 a pronounced influence upon the growth of all or- 

 gans exposed to them. For example, the continual 

 pull due to the weight of a leaf or branch must 

 react upon the growing cells of the organs and in- 

 fluence the development of tissues. 



The Necessary Chemical Constituents of Plants. 

 The growth of plants is dependent on the pres- 

 ence of certain chemical elements which are indis- 

 pensable as food, or are in some way bound up with 

 the constructive processes going on in the plant. 

 The growth of the plant is often very greatly af- 

 fected by the presence or absence of certain elements 

 which form a very insignificant part of its sub- 

 stance, but which are evidently indispensable in the 

 processes of growth. Thus a plant cultivated in an 

 artificially made nutrient solution containing all the 

 essential chemical elements except potassium or iron, 

 will show very little growth, although the amount 

 of iron or potassium present in the tissues is ex- 

 cessively small; but if entirely deprived of these ele- 

 ments the plant is quickly dwarfed and growth will 

 be almost entirely suspended. 



The movements of motile plant cells may be mark- 

 edly affected by the attractive power of certain sub- 

 stances, and this chemical attraction has been named 

 Chemotaxis. Thus many bacteria will quickly 



