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releasing the stored energy in available form for the constructive 

 work of the organism is as necessary in plants as it is in animals. 

 These four fundamental questions, namely, the inorganic sub- 

 stances required by plants, the manner of their absorption, the 

 manufacture of the first organic food, and the nature of respira- 

 tion are perhaps the most important physiological facts, in the 

 field of nutrition at least, which have been definitly established, 

 and from any point of view their importance is a far reaching one. 

 In the other great field of physiological research, the study of 

 the mechanism of growth and change of form, much informa- 

 tion, made possible by the proper understanding of the cellular 

 character of all living organisms, has established many facts as 

 to the relation of plants to the great physical forces which govern 

 the conditions, the rate and the direction of their growth. This 

 is the study of the dynamics of plants, of when and how the 

 energy released by the nutritive functions is applied to the up- 

 building of new tissue and the movement of plant organs. Be- 

 sides the questions concerned in the influence of diffusely exerted 

 external factors, there are also the effects produced by these 

 same forces when the stimulus is unequal or one-sided. The 

 latter conditions result in characteristic growth curvatures or 

 tropisms, which continue until the plant organ by its own action 

 is brought once more into a state of equilibrium with the external 

 forces. In short, the various plant organs are attuned to the 

 normal conditions of equilibrium under which they grow, and 

 have the ability to perceive and, to a limited extent, to transmit 

 the impulses resulting from a disturbance of that equilibrium. 

 This brings us to the question of the sense perception of plants, 

 manifested in a somewhat bizarre fashion in the sensitive plant, 

 but we should go very slowly in the direction of interpreting this 

 perception in the same terms that we do that of higher animals. 

 It is not for an instant to be supposed that plants have any nerv- 

 ous system such as is characteristic of the higher animal forms. 

 While plants can and do respond to differences in light intensity 

 less than that which the human eye can perceive, it is gratuitous 

 to suppose that there is anything analogous in the two processes. 

 The possibility of any reasoning action or instinct on the part 



