68 TRANSPIRATION AND ASCENT OF SAP ch. 



appeared on the fifth leaf from the base, while the seven 

 leaves above were quite healthy. On A there were five 

 leaves dead ; on B two small ones and two large leaves 

 blotched, on C there was one withered and crisp, and one 

 blotched. 



At this stage, 14 cm. at the base of C was heated, the 

 leaves above being protected. 



On the next day all the leaves of C were stained and 

 beginning to curl. No further changes were noticed in 

 A and B. 



Six days after the heating of C all its leaves were dead, 

 while still the uppermost leaf of D was unaffected, although 

 its supply was drawn through 22 cm. of stem which had 

 been killed twenty days previously. 



In this experiment, evidently the flushing of the killed 

 branch removed much of the deleterious substances from 

 the supply to D, for its changes were at first less notice- 

 able than those of the branches below, into which evidently 

 the major part of these substances were distributed. The 

 changes observed in C after its base was heated, indicate 

 the extent and the rapidity of injury we might have 

 expected in D had the killed region not been washed 

 out. 



Thus we see there is a large mass of experimental evi- 

 dence showing that the fading of the leaves on a killed 

 stem is due to the introduction of poisonous or plasmo- 

 lysing substances into the transpiration sap. In these 

 branches the freedom of motion of the upward water 

 current may be reduced by plugs formed above the heated 

 region and in the veins of the leaves themselves ; but, if 

 sufficient length of the stem has been killed, poisoning 

 will apparently always supervene, whether plugging occurs 

 or not. With these facts established, this fading affords 

 no evidence in support of the view that the vital activities 

 of the cells of the stem are essential to the raising of the 

 transpiration stream. 



