PREFACE TO VOLUME II. 



I HAVE in the present volume dealt with the intricate 

 phenomena of different tropisms. The movements in plants 

 under the stimuli of the environment — the twining of 

 tendrils, the effect of temperature, the action of light 

 inducing movements sometimes towards and at other times 

 away from the stimulus, the diametrically opposite responses 

 of the shoot and the root to the same stimulus of gravity, 

 the day and night positions of organs of plants — these, and 

 many others present such diversities that it must have 

 appeared a hopeless endeavour to discover any fundamental 

 reaction applicable in all cases. It has therefore been 

 customary to assume different sensibilities especially evolved 

 for the advantage of the plant. But teleological argu- 

 ment and the use of descriptive phrases, like positive 

 and negative tropism, offer no real explanation of the 

 phenomena. Thus to quote Pfeffer "When we say that an 

 organ curves towards a source of illumination, because of 

 its heliotropic irritability we are simply expressing an 

 ascertained fact in a conveniently abbreviated form, without 

 explaining why such curvature is possible or how it is 

 produced. . . . Many observers have unfortunately devoted 

 their attention to artificially classifying the phenomenon 

 observed, and have entirely neglected the explanation of 

 causes underlying them." He also adds that in regard to 

 the phenomenon of growth and its variations, an empirical 



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