48 Prof. H. H. Dixon. [Nov. 7 



ordinates of the curves there shown indicate weights of filtered tap-watei 

 transmitted through 3 cm. of wood of Abies pectinata per second. The 

 abscissae indicate time in minutes. The curves show the diminution in the 

 rate of transmission for each pressure, the fall-off in the amount being much 

 more rapid for the higher pressures. From these it is seen that it is desirable 

 (especially when using colour solutions, which from the nature of the case 

 cannot be distilled) to employ low pressures in order to determine the 

 resistance of the conduits apart from the surface resistance. For this reason, 

 then, when I began to determine the resistance of the tracheidal tubes I 

 preferred to experiment with very low pressures, assuming that the flow for 

 higher pressures would be proportional 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 far the most suitable wood for experiment, when 

 the qualities of the water-conducting tissues 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 constituents 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 materially 

 differ in percentage from one level in the stem to another, and consequently 

 do not introduce differences of serious magnitude. The fact, however, that 

 they exude small amounts of slimy materials sometimes introduces 

 difficulties. But inasmuch as there are approximately the same amount of 

 medullary ray-cells per unit area in every cross-section, the clogging 

 introduced in this manner is uniform and, it is to be noticed, tends to 

 increase the apparent resistance. The presence in other woods of large 

 medullary rays, of wood-parenchyma, of vessels of variable size, or even of 

 resin ducts, renders them unsuitable for experiment. 



Certain precautions are necessary in preparing the wood for these 

 transmission experiments. It is best to cut away the transpiring leaves 

 from the selected branch, and then to cut off a short length of it (say 15 to 

 25 cm.) under water. Five to ten centimetres are now cut from each end, 

 and the remainder, after lying in water for at least 30 minutes, may be used 

 directly or reduced to smaller lengths for experiment. These precautions 

 are necessary, so that bubbles shall not be generated in the conduits. It 

 seems possible that Ewart's high estimate of the resistance was, in part, due 



