PHYSIOLOGY 161 



Vital phenomena are essentially bound up with the living 

 protoplasm. No other substance exhibits a similar series of remark- 

 able and varied phenomena, such as we may compare with the 

 attributes of life. As both physics and chemistry have been restricted 

 to the investigation of lifeless bodies, any attempt to explain vital 

 phenomena solely by chemical and physical laws could only be induced 

 by a false conception of their real significance, and mustlead to fruit- 

 less results. The physical attributes of air, water, and of the glasses 

 and metals made use of in physical apparatus, can never explain 

 qualities like nutrition, respiration, growth, irritability and repro- 

 duction. It would, indeed, be superfluous to emphasise the fact, were 

 it not that this error is from time to time repeated. 



The phenomena of life can only be studied and determined by the 

 most careful observation and critical examination of living organisms. 

 It is therefore necessary to establish what part the purely physical and 

 chemical properties, which belong to all bodies, take in the phenomena 

 of life, and to what extent they are essential to the maintenance of 

 life itself. A perception of the strictly physical and chemical processes 

 going on within an organism is especially desirable, because operations 

 are then involved with the causes and effects of which we are already 

 familiar. In questions regarding strictly vital phenomena the case is 

 quite different ; for it then becomes impossible to predict what effect 

 a particular cause will produce. The free end of a horizontally 

 extended flexible rod bends downwards merely by its own weight. 

 The same result will follow if any part of a dead plant, such as 

 a dry stem, be substituted for the rod. But if a living, growing- 

 stem be used in the experiment, then the action of gravity will 

 manifest itself in a manner altogether at variance with its ordinary 

 operation. That part of the stem which is still in a state of growth 

 will ultimately curve upwards, and BY its OWN activity assume an 

 upright POSITION ; it moves in a direction exactly contrary to the 

 attractive force of gravity. If a tap-root be similarly experimented 

 upon, it will, on the contrary, continue its downward movement until 

 it places itself in a line with the direction of the attraction ; a rhizome, 

 however, under like circumstances, would constantly maintain its 

 growing apex in a horizontal position. 



In these three experiments the force of gravity is exerted upon 

 flexible portions of plants. The physical conditions are the same in 

 each case, yet how entirely different the results ! 



The explanation of this dissimilarity in the effects of the action 

 of gravity is to be sought in the fact that gravity acts upon liv- 

 ing substances, not only physically but also in another way, as a 

 stimulus which induces a response in the internal forces of the plant 

 body. In these particular experiments it is the force of growth 

 which, locally, either increases or restricts the force of gravity, and 

 produces results which do not correspond either qualitatively or 



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