is as necessary in plants as it is in animals. These four 

 fundamental questions, namely, the inorganic substances 

 required by plants, the manner of their absorption, the 

 manufacture of the first organic food, and the nature of 

 respiration are perhaps the most important physiological 

 facts, in the field of nutrition at least, which have been 

 definitely 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 information, made possible by the proper under- 

 standing 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. 

 Besides the questions concerned in the influence of dif- 

 fusely exerted external factors, there are also the effects 

 produced by these same forces when the stimulus is un- 

 equal or one-sided. The latter conditions result in charac- 

 teristic 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 equilib- 

 rium. This brings us to the question of the sense percep- 

 tion 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 nervous system such as 



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