Transpiration and the Resistance of Stems 343 



the conductivity of stems. In the first example the stem above 

 the potometer for about 35 cm. was covered with a layer of snow 

 about 2 cm. thick and bound with sheet rubber so that there could 

 be no drip upon the potometer. A thermometer was placed among 



the leaves, 5 cm. above the jacket of snow in order to determine 

 whether any variation of rate could be explained by a lessened 

 action of the transpiring organs rather than by the increased re- 

 sistance of the stem. The snow was applied for about one half 

 hour, producing a fall of temperature among the leaves of only 2.8 

 degrees. The wide variation from the normal transpiration rate 

 of 16.4 to 36.2 cannot of course be attributed to so slight a fluctu- 

 ation of temperature and the delayed approach to the normal rate 

 also clearly indicates that the greatly reduced temperature of the 

 stem is the chief factor in the reduction of the rate. It is apparent 

 that the wide variation is not at all in keeping with the fluctua- 

 tions to be noted in capillary tubes and the marked changes of 

 rate may probably be accounted for as principally due to reduction 

 in the size of the cell cavities. 



V. 



Abut Hon 16 12 50 20.5 19.6 I 10 18.6 35 I 25 17-7° 34-6 i 40 19-4 



16 8 20.2 3 6 - 2 33- 8 



16.5 21 36 34-5 

 16 22.5 37 33-4 



16.4 23 36.4 3 2 

 Citrus 25 1 120 !8.3° 28. II 45 15.5° 33.5 12 16.5 27 12 20 19.4 



26 28.6 33.5 26 



25.6 31. 33.5 25 

 26.2 32. 33 25.5 

 26 32.5 33 2 4- 



Abutilon 55 2 20 23 I 20 2 30 8° I 25 2 5u 20.6 



56 1* I 20 



56 1* i 11 



57 1 34 i 6 

 56 i 3 * i 2 22 



Abutilon 54.3 3™ 24.7 I 10 3 40 8° I 14 3 50 



54-5 I 13 l8 



55 I- 15 *** 



54.5 I 18 l7 



545 I 19 l6 



The branch of Citrus was treated in the same way, about 20 cm. 

 of the stem being covered with snow, with practically the same 

 results. It is to be noted that the rise of temperature in the room 



12° 



13 



4 20 



21° 



14° 



1 



58 

 58 





23° 



[6° 



56 





24° 



