173 



should dimiuisli towards tlie edges as represented in the accompan}'- 

 ing sketch. The apparatus need not be present in cells which are 

 insensible to stimulation. 



The normal vertical position of tiie main axis and radicle would 

 seem to imply that in these organs the middle-field lies against the 

 basal transverse wail of the statocyst. But in a horizontally-growing 

 rhizome, for instance, it ought to ly next lo the lower longitudinal 

 wall, for then only there would be maximal stiniulus, accompanied 

 l)v the maximum, equal all round, speed of growth, whereby the 

 rhizome would keep its horizontal position. 



If now a certain shifting of the static apparatus is required to 

 produce a new position of equilibrium, then inversely we might 

 deduce from the changement in the position of equilibrium what 

 shifting should have taken place in each separate case, but therefore 

 it were necessary to know also in what part of the oigan the static 

 apparatus occurs. This shifting cannot be microscopically contiolled, 

 for the present at least, but if it should appear from the following 

 lines that by assuming such a shifting we succeed in giving a simple 

 explanation of widely different and often very complicated pheno- 

 mena, this must favour our supposilion of the presence oï&inovable 

 excitable organ in the sensitive cell. It must be borne in mind, 

 however, that it i.^ therewith immaterial whether we think of an 

 "static apparatus" as indicated above, or of the outer layer as a 

 whole, provided this be but movable; in future we shall suppose 

 the presence of a 'static apparatus". 



If it be possible by this means to explain why a plant-organ 

 which has a certain position of equilibrium is able to keep this 

 position during its growth, it does not, however explain the familiar 

 |)henomenon of an organ (hat is brought out of its equilibrium 

 returning to this position, not only of its own accord, but also by 

 shortest jiossible way; so a root, for instance, placed horizontally 

 will cui've downwards in a vertical [)lane until the tip points per- 

 pendicularly again. That this movement is of great advantage for 

 the later development of the plant is of course no sufficient ex|)la- 

 nation of its cause, especially since the preparations for the movement 

 are made long before the utility of the bending could be perceived 

 by the plant. We should have lo ask, therefore, why it is that a 

 part of the plant makes a useful movement and how it comes thai 

 the new position is acquired by the shortest way. 



This question which, as it seems to me, is proposed here for the 

 first time so sharply, is connected so deeply with the more intimate 

 life of the cell that it can not surprise that no entirely complete 



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