54 TRANSPIRATION AND ASCENT OF SAP ch. 



hot water, it is found that the leaves above the killed 

 portion sooner or later fade and wither. The vitalists' 

 interpretation of this observation is that, when the vital 

 actions of the wood parenchyma and medullary-ray-cells 

 are removed, the supply of water to the leaves above is 

 so reduced that they fade and dry from want of water. 

 It is significant that the discoverers of the phenomenon 

 saw in it no support of the vital hypothesis. Weber 

 recognises that the reduction of the water supply may be 

 attributed to the stoppage of the tracheal tubes. Janse, 

 although supporting Godlewski's vital theory, quickly saw 

 that this observation, being traceable to the blocking of 

 the supply tubes, could not be quoted in support of the 

 vital hypothesis. Finally, Vesque sees no support in it 

 for the vital theory. Ursprung, however, one of the 

 most recent champions of this hypothesis, relies on the 

 observation with great confidence to support his view, but 

 even his observations show that, out of some twentv 

 species in which the phenomenon was observed, in fourteen 

 stoppages after heating in the conducting tubes were actu- 

 ally observed usually above the heated region. Ursprung, 

 in spite of this, maintains that the reduction in the water 

 supply is not caused by stoppage because (1) the leaves 

 may begin to die before the stoppage of the tubes is ob- 

 served ; (2) even when stoppages do occur, fading may 

 be postponed ; (3) fading does not occur when a piece of 

 the wood is removed from an uninjured branch corre- 

 sponding in size to the plugged portion of the heated 

 branch. 



Stoppages in the tracheae. It is to be noted 

 that the failure to observe stoppages microscopically 

 does not negative their existence. A transparent material 

 might form quite an effective plug in the tubes, or 

 render the transmitting pits almost impermeable, while 

 it might be almost or quite invisible. Such a trans- 

 parent substance Weber did actually detect in his experi- 



