3/2 COMPARATIVE ELECTRO-PHYSIOLOGY 



enhanced by a rise, and depressed or arrested by a fall, of 

 temperature. A similar effect is seen to occur in the suctional 

 response of plants. Thus, in a given specimen of Croton, 

 application of cold water at 4 C. to the root was found to 

 arrcsTtEe suction in the course of 8 minutes^ This arrest 

 by o>ld was not permanent, for the normal rate of suction re- 

 appeared on the return of the water to a normal temperature. 

 On applying water of raised temperature to the root, on the 

 other hand, the rate of suction was immediately found to 

 be enhanced. This is illustrated in the following record 

 (fig. 219), in which the normal rate of suction at 23 C. was 



7 cubic mm. in volume, or 7 mg. 

 in \vcight_ per minute. The ap- 

 plication of water at 35 C. now 

 induced a steep rise of the curve, 

 indicating an enhanced rate of 

 suction of 58 mg. per minute, or 

 more than eight times the rate at 

 J3 C. On now once more sub- 

 stituting water at 23 C., the rate 

 is seen to become lowered, but 

 not to fall so low as at the 

 beginning. It now fell from 58 



Suction at 35 C., and the to 14 mg. per minute, instead of 

 ^'Si^ RetUrn returning to the original 8 mg. 



This is due to the fact that 



the internal energy of the tissue has been raised in the 

 meantime by the absorption of warm water, the enhancement 

 of the normal rate being a persistent after-effect of this. 



We shall next take up the apparently anomalous case, 

 in which, when the root has been killed by pouring boiling 

 water over it, the suction of the plant is nevertheless main- 

 tained. In such an experiment the normal record was first 

 taken, and boiling water was then passed through the plant- 

 vessel continuously for some time till the roots were killed. 

 On allowing the water in the vessel to return to the tempera- 

 ture of the room, it was found that suction was taking place 



