CHEMO-TROPISM AND GALVANO-TROPISM 549 



Action of sugar solution. — A dilute solution of this agent 

 has already been shown to accelerate growth. Its unilateral 

 application was found, in the present case, to bring about 

 convexity of the proximal side of the specimen ; but very 

 strong solutions of sugar were found, as we have seen, to 

 retard growth, and the unilateral application of such a solu- 

 tion was now found to induce concavity of the proximal side. 



Chemo-tactic movements. — We have now seen that the 

 excitatory or depressing action of various agents is indicated, 

 in the case of the growing organ, by a curvature one way or 

 the other. Similar movements are induced also in pulvinated 

 organs ; but in organs which are capable of multiple re- 

 sponse we shall expect to find that the corresponding effects 

 induced by such agents will consist in similar movements 

 often repeated— that is to say, in the initiation of such 

 multiple response, or in its appropriate modification. It will 

 be shown in Chapter XLIX. that the swimming movements 

 executed by ciliated organisms form an instance of such 

 multiple response. It will further be shown in that chapter 

 that the organs respond to stimulus, whether chemical, 

 photic, thermal, or electrical, by swimming either towards or 

 away from it. 



Explanation of anomalous osmotic or plasmolytic 

 action. — An attempt has sometimes been made to explain 

 the responsive movements of plant-organs as the result of 

 supposed osmotic variations within the tissue. Thus it was 

 held by De Vries that the growth-curvatures of multi- 

 cellular organs were brought about by an increase of osmotic 

 substances on the convex side, giving rise to augmented 

 hydrostatic pressure. It was afterwards ascertained, how- 

 ever, that the convex side of the curved organ did not con- 

 tain any greater quantity of osmotically active substances 

 than the concave. The method of plasmolysis of De Vries 

 is often used for the determination of the differential osmotic 

 activity and turgescence on the two sides of a curved organ. 

 After plasmolysis, the curve induced in the organ is flatter 

 than before, and from this it has been inferred that the cell- 



