126 TRANSPIRATION AND ASCENT OF SAP ch. 



assuming that the flow for higher pressures would be pro- 

 portional to the head. This point was examined later. 



The wood used in these experiments was that of Taxus 

 baccata. The uniformity and comparative homogeneity of 

 structure (resulting from the fact that it is composed 

 solely of tracheids, and is not penetrated by resin ducts) 

 recommend it as by far the most suitable wood for experi- 

 ment, when the qualities of the water-conducting tissues 



6P 70 80 



Minutes 



Fig. 19. 



are being examined. The tracheids composing the wood 

 of Taxus baccata are elongate spindle-shaped chambers 

 1 to 5 mm. long, and approximately square in cross- 

 section. The cavity of these chambers is comparatively 

 small. In cross-section the lumen forms about a quarter 

 the entire area of the tracheid. The only other consti- 

 tuents of the wood are the cells of the medullary rays, 

 which are radial tiers of cells 0'4 to 0'02 mm. in height. 

 They do not differ materially in percentage from one level 



