vi TENSION REOUIRED TO RAISE THE SAP 127 



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 

 causes difficulties. But inasmuch as there are approxi- 

 mately 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 unsuit- 

 able for experiment. 



Preparation of material. 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 experiments 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 to bubbles being generated in the conduits 

 while preparing the wood for his resistance experiments. 

 He certainly does not mention having taken these or 

 similar precautions in this connection, and his results are 

 three to four times as high as those given when these 

 precautions are taken. 



Measure of the resistance. When determining 

 the velocity of flow with colour solutions, I usually 

 worked with a head of water equal to, or less than, 

 the length of the piece of wood experimented with. 

 When using unit head {i.e., the head equal to the 

 length of wood used), a fresh surface was cut with a 

 razor on one end of a piece of a branch prepared as just 

 described. The piece was supported vertically with the 



