348 Curtis : Work Performed in Transpiration 



of plants and among plants of the same species. Various parts of 

 a stem differ widely in the resistance offered and consequently 

 there is no ratio in the rate of transmission of a stem under pres- 

 sure and the length of stem. 



The rate of transmission is not affected by electric currents and 

 various solutions as in capillary tubes. The former has no effect 

 while the relation of the character of the wall to the nature of the 

 solution may have a marked effect in the rate of transmission. 

 Changes of temperature produce marked fluctuations in the 

 amount of resistance, due largely to variations in the lumen area 

 of the cells. 



Partial severing of the water conduits by cuttings does not pro- 

 duce variations in the rate corresponding to those brought about 

 by constricting the bore of tubes at a given point. On the con- 

 trary the resistances so introduced are practically equivalent to an 

 increase in the length of the stem proportional to the extent of the 

 cutting. Only a portion of the lumen area is utilized by the plant. 

 This condition may be maintained within certain limits either in 

 slowly or actively transpiring plants through variations in the rate 

 of transmission. 



