CHAPTER XIX 



EFFECT OF COMPLEX AGENTS UPON GROWTH, AND 

 GENERAL CONCLUSIONS 



IN the foregoing chapters we have examined the effect upon 

 growth of various agents considered as acting separately. This 

 treatment has been necessary for purposes of analysis, but it 

 has the disadvantage that it does not reveal the normal 

 action of these agents. For, in nature, we find many agents 

 working together upon the growing organism. For example, 

 gravity is constantly acting in one direction, upon sessile 

 organisms at least, and at the same time the chemical character 

 and the density of the surrounding medium, contact and im- 

 pact, light and heat, are exerting their specific influences. The 

 rate of growth of an organism and the direction of growth of 

 its organs are determined by the resultant of some half dozen 

 controlling factors which, in their totality, constitute environ- 

 ment. 



Again, some of the factors influencing growth are so complex 

 that they are not yet amenable to analysis into the chemical, 

 molar, and physical agents whose effects we have considered in 

 foregoing chapters. For instance, the reaction of an organism 

 to its own activity is still too complex an effect for us to be 

 able to resolve it into its elemental reactions to pressure, chem- 

 ical change, etc. The effect of exercise upon growth may con- 

 sequently form a subject for special consideration. 



1. THE COOPERATION OF GEOTROPISM AND PHOTOTROPISM 



When light falls from one side upon a seedling in the ground, 

 the seedling is influenced by both gravity and light, and tends 

 to respond to both. In responding to both, the apex of the 

 seedling tends to point upwards on account of gravity's action, 

 and laterally towards the source of illumination on account of 



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