MODIl'ICATION OF SUcTIUNyVL RKSPONSK 373 



tent, by subjecting- the plant to the action of an agent whose 

 general effect is well known, and recording the results by all 

 three methods. For this purpose we shall take the influence 

 of low and moderately high temperatures. 



Modification of suctional response by various agencies. 

 We have seen that any sudden variation of temperature 

 acts as a stimulus in itself Thus, if we touch the pulvinus 

 of Mimosa or BiopJiytum with ice, there is a responsive 

 twitch. This may be taken as the preliminary effect. Pro- 

 longed application of cold, however, abolishes excitability. 

 If the ascent of sap be really a phenomenon of excitation, 

 we may expect to find a sudden application of cold, appro- 

 priately made, producing a preliminary augmentation, followed 

 by the depression and arrest of suction. An application of 

 hot water might be expected, on the other hand, to bring 

 about the contrary effect, that is to say, an increase in the 

 rate of suction. I shall now describe the experimental results 

 obtained by the three methods. 



Effect of temperature on suction : (i) Unbalanced 

 Method, id) Action of cold. — As I did not know what might 

 be the effect of injury on the suctional activity of the plant, 

 I selected intact specimens for my first experiments. For 

 this purpose I took cuttings of Croton, a plant whose stem 

 when placed in water will develop roots in a {^.^n weeks' 

 time. When the roots were well developed, the specimen 

 was fitted in its place in the apparatus. In other cases, 

 I took pot-grown specimens and placed them in water, so 

 that the earth was dissolved away. 'Violence to the rootlets 

 was thus avoided. I may here state, however, that I found, 

 in the course of these experiments, chat there is no essential 

 difference between the effects exhibited in such intact plants 

 and those observed in' the case of cut branches. All that is 

 necessary in the latter case is that the specimen should be 

 mounted in the apparatus and left for some time, in order 

 that the effect of the disturbance caused by cut may pass. 

 The record afforded by a specimen thus mounted gives the 

 normal rate of suction. The attainment of constancy of 



