Transpiration and the Resistance of Stems 



345 



Taxus 6. 7 



7 

 6.8 



7 



8 



*3-6 15.5 9 



7-5 13-9 

 7-5 



12.2 



15-5 

 16 



7 2 13.8 15.6 



7.2 



7.2 



7-1 



12 



151 



7.1 11. 6 15.6 



7 

 7 

 7 

 7 

 7 

 7 



2 58 15-6 34 52 8 7Xi-o 



4 

 4 

 4 

 4 



36 

 36 



5 



37 

 18.5 36 



17 

 18 



17 



18.2 



58 



59 



59 

 60 



61 



The second part of the table gives the results when lengths 

 of stems under pressure were treated in the same way. The 

 number to the 



right 



of the first column indicates the pressure 

 required and the three succeeding columns record the results of 



cutting y 2 and ^ through and to a few sq. mm. The final num- 

 bers give the dimensions of the stem. The lengths of these 

 stems also indicate the distance above the potometer at which the 

 cuttings were made. 



These cutting experiments of Dufour are among the most im- 

 portant that can be performed in transpiration since they give a 

 very clear demonstration of the vital action of the living cells in 

 maintaining the transpiration stream. The recovery from the 

 effect of cutting noted in the first part of the table is entirely done 

 away with in the second series of measurements upon the stems 

 under pressure. In fact the results in this latter case are practi- 

 cally the same as those obtained in the different lengths of stems 

 in Table II. These results were certainly not to be expected and 

 may be accounted for, I believe, on the ground that the velocity 

 of the fluids at the point of cutting is so enormously increased that 

 a resistance is generated quite out of proportion to that acting on 

 the stem at the normal rate and sufficient to reduce the rate prac- 

 tically to that of stems increased in length in proportion to the 

 depths of cutting. 



Relation of Conducting Area in Stem to Volume of 



Fluid 



The ability of the plant to maintain the water current unchecked 

 through so small an area of stem led to some investigation upon 

 the extend of the area at the disposal of the plant for the conduc- 

 tion of fluids and the precentage of this space actually utilized. 

 To determine the percentage of cavity in the stem microscopic 

 cross sections were prepared and areas, representative of the various 

 growths of the stem were drawn on sheets of tinfoil with a camera 



