in ASCENT OF SAP IN STEMS 51 



Evidence from structure. The very structure of 

 the conducting wood of trees, far from supporting 

 Godlewski's contention that the cells assist in 

 elevating the transpiration stream, offers the strongest 

 evidence against it. These cells, placed as they are 

 beside the transmitting tubes, can, by their pumping 

 actions, in no way exert a lifting force on the stream. 

 The water, as experiment shows, is free to move down- 

 wards as well as upwards. Nor would a polarised or uni- 

 directional action of these cells, as hypothecated by Janse, 

 help, owing to their relation to the tubes. In that case, 

 these cells would regularly draw in water on one side and 

 expel it on the other into tubes, where it is exposed both 

 to downward forces and to resistance to upward motion. 

 In fact, to utilise the pumping action of living cells in 

 raising the transpiration current would require that the 

 continuity of the conducting tracts should be here and 

 there completely interrupted by the pumping cells ; but 

 if flow be possible in permeable tissues round these groups 

 of cells, pumping actions on the part of the cells will be 

 futile in assisting to raise water. As is well known, no 

 such interpolation of cells cutting the continuity of the 

 woody tissues is revealed by the most careful study of 

 the conducting tracts. The structure of the latter 

 tissues is, therefore, fatal alike to the earlier views of 

 Godlewski and Westermaier, and to the less precise form 

 under which they have been recently resuscitated by 

 Ursprung. 



Exception has been taken to the statement that the 

 structure of the wood is against the vital hypothesis, and 

 it has been maintained that this statement is based on 

 ignorance of the resistance offered by the wood to the 

 nitration of water. 



Assuming the resistance to be considerable, Janse be- 

 lieves that a pumping action in the medullary-ray-cells is 

 able to account for the rise of the water. This action, 



e 2 



