EFFECTS OF DRUGS ON PLANTS 485 



as will be seen from the record, of neutralising the previous 

 contractile arrest, and in the course of two minutes it had 

 brought about the renewal of growth. 



Action of strong solution of sodium chloride.— In the 

 course of the investigation on suctional response we noticed 

 that the effect of this reagent, when applied to the root, was 

 not always to cause a diminution of suction, as might have 

 been expected had osmotic action been the only factor. This 

 reagent, on the contrary, usually operated to bring about a 

 preliminary acceleration of suction. And I have already ex- 

 plained that this was due to its excitatory character, acting in 

 opposition to the osmotic action set up by the strong solu- 

 tion. It is usually supposed that the excitatory effect of 

 strong solution of salt on animal tissues is due to the osmotic 

 withdrawal of water ; but we have here seen that similar 

 excitation is induced in vegetable tissues without any with- 

 drawal of water. Hence the usual theory of the action of 

 salt solution in causing excitation is rendered very doubt- 

 ful. We found, in fact, that when the tonic condition of the 

 tissue was favourable, the excitatory reaction predominated, 

 and there was a consequent enhancement of suctional 

 activity. In other instances, where the tonic condition was 

 less favourable, osmotic action predominated, and there was 

 a consequent depression of the normal rate of suction (p. 385). 

 It is thus seen that the factor of variation in these two 

 different cases was the constitution, or tonic condition, of the 

 plant ; we are now able to study with precision the influence 

 exercised by constitution, in causing the plant to struggle 

 against, or succumb to, the action of adverse external cir- 

 cumstances. The action, in causing variation of suction, of a 

 strong solution of salt, applied internally through the cut end 

 of the stem, can also be studied by means of growth-response ; 

 for while increased suctional activity will give rise to a posi- 

 tive turgidity- variation, with concomitant enhancement of the 

 rate of growth, diminished suctional activity will have the 

 opposite effect, of retarding the rate of growth. We shall 

 next observe the effect of this reagent on different specimens 



