3 88 PLANT RESPONSE 



Summary 



Rhythmic activity being as a rule exalted by rise of 

 temperature, suctional response, as one of its effects, also 

 undergoes an increase. 



Suction being an expression of excitatory response, the 

 direction of the resultant movement of sap is determined by 

 the relative excitabilities of the two ends of a tissue. Under 

 certain circumstances, the normal direction of movement of 

 sap may be reversed. 



The application of cold produces a transient excitation, 

 and thus causes a preliminary enhancement of suction. Pro- 

 longed application of cold, causing a depression of excit- 

 ability, brings about arrest of suction. 



As suction is produced by the rhythmic activity of the 

 tissue of the entire plant, local death, as by scalding or 

 application of poison, does not cause its arrest, until the 

 whole plant is killed. 



When the sum total of the latent energy of the tissue — 

 that is to say, its tonic condition — is below par, its rhythmic 

 suctional activity comes to a stop ; fresh application of 

 stimulus, however, renews this activity. 



Osmotic substances, as regards their stimulatory action, 

 may be either neutral or excitatory. If such a solution be 

 applied at the root, there will in the former case be a diminu- 

 tion, and in the latter, if the excitatory action be relatively 

 great, an increase of suction. 



The application of poison abolishes local excitability and 

 power of suction. In some cases this arrest of suction may 

 occur quickly. But the total abolition of suction by poison 

 only takes place, for reasons already explained, on the death 

 of the plant as a whole. 



