in ASCENT OF SAP IN STEMS 55 



merits on Picea excelsa. This investigator's observation 

 that the resistance of the region immediately above the 

 killed part is enormously increased, is a piece of positive 

 evidence in favour of stoppage which cannot be put aside. 

 He found that the lower part (1T8 cm. long) of the heated 

 stem, including both boundaries between the dead and 

 the living regions, refused to transmit water under a head 

 of 62 cm. of mercury. Microscopic observation shows that 

 clogging is greatest in these border regions. The perme- 

 ability of the upper parts of the same stem was much 

 greater, as was seen from the fact that the upper 16*5 cm. 

 supporting the faded leaves transmitted sufficient water 

 under a head of 59 cm. of water to render the leaves 

 turgescent once more. In several other experiments Weber 

 has shown that the resistance of the conducting wood is 

 enormously increased where the uninjured part borders on 

 the heated region. Janse confirmed and extended Weber's 

 observations of this increase of resistance even when the 

 temperature to which the branch was exposed was as low 

 as 60-64. 



Tt is easy to show that the water traversing a piece of 

 stem killed with heat is contaminated in its passage. 

 Thus, if distilled water is forced through a piece of a 

 stem freshly cut from a tree, it is transmitted without 

 sensible coloration so long as morbid changes in the stem 

 do not take place. But if the same stem is surrounded 

 with steam so that its cells are killed, the water which 

 emerges is no longer colourless, but is tinged, more or less 

 deeply, with brown. When attached to the tree the 

 killed piece must contaminate the rising transpiration 

 stream in the same way. The record of this contamination 

 first appears on the walls of the tracheae of the leaves, 

 which become stained ; then in their lumina, which become 

 filled with the coloured fluid concentrated by evaporation. 

 This substance is finally caught in the walls of the con- 

 ducting tubes of the stem and forms plugs, filling their 



