722 PLANT RESPONSE 



latent, in whole or in part, for subsequent expression. ' Inner 

 stimuli ' are simply external stimuli absorbed previously, and 

 held latent. A plant or an animal is thus an accumulator, 

 which is constantly storing up energy from external sources ; 

 and in the case of the plant, its suctional activity, determining 

 the ascent of sap, its growth, and its spontaneous motile 

 indications, are some of the principal forms in which this 

 accumulated energy finds expression. 



The ascent of sap. — The ascent of sap has been shown 

 to be due to a multiple excitatory reaction of the plant- 

 tissue, the movement of the water being a secondary effect 

 of the rhythmic activity. The excitatory nature of the 

 phenomenon has been demonstrated by the fact that various 

 agencies which induce increase or diminution of excitability, 

 have also the effect of bringing about an enhanced or 

 diminished rate of suction, above or below the normal. The 

 effects induced by these agencies, together with their time- 

 relations, can be easily and accurately recorded, as has been 

 shown, by means of the Balanced Shoshungraph. The transient 

 excitation due to a sudden application of cold, and the 

 abolition of excitation under its prolonged application, are 

 seen in a transient enhancement of suction, followed by 

 arrest. The excitatory effect of the application of hot water, 

 again, is shown by an enhanced rate of suction. Poisonous 

 chemical reagents arrest suction quickly in specimens where, 

 owing to a less favourable tonic condition, the power of 

 resistance is low, and slowly in other cases. As. the tissue of 

 the plant exhibits this suctional activity throughout its length, 

 the local death of a given portion, by scalding or by poison, 

 would not necessarily arrest the suction of the entire plant. 

 Such an arrest can only occur definitely when the entire 

 plant is killed. 



The internal energy, on which the activity of suction 

 depends, may fall so much below par as to bring it to a 

 standstill ; but the activity is renewed on the application of 

 fresh stimulus. 



That the ascent of sap is not fundamentally due to 



