Transpiration and the Resistance of Stems 



337 



Abu til on 



a 



a 



i i 



Taxus 



Ta 



xus 



(I 



<< 



u 



63-5 



!58 

 I »* 



,M 



1 22 



{tt 



! 21 



I 2 ' 

 350 



2 40 



65 41 10 X -7 



2' 2S 20 IO X I.O 



2 5 6 28 IOX -7 



i 36 38 10X -8 



i' 9 48 10X -8 



T 21 



!20 

 1 22 



i 23 



3 45 

 2 43 



Acalypha 48 



63 

 6 



8 



Salix 



Ailanthus 

 Corn us 



47 

 65 



7 

 7 



32 

 26 



53 

 29 



7-5 6.5 71 



10X -8 

 10X -9 



10 x 1.0 



10X -9 

 10 X 9 



25 10 X -6 



42 10 X -6 



12 105 10 X IO 



59 58 52 10X -8 



12 



8 

 8 



7 



7 

 8 



13 



12 



13 



Viburnum 73 



36 



7 



7 

 8 



8.5 

 9 



Cornus 



a 



n 



13 

 13 



71 



38 



39 



Si 



48 



38 

 64 



10 X i-o 



10X -8 



10X -7 

 10X .6 

 10X i-o 



The experiments given in the first table are examples from a 

 great number that have been performed in our laboratory and are 

 representative of the variation and extent of the resistance that 

 the transpiration current must overcome. The stems were usu- 

 ally tested after standing over night in water or less commonly 

 three or four hours in water. In the first column opposite the 

 name of the plant is recorded the transpiration rate. In four 



iyph 



and Cornus, six consecutive 



readings are given to show the variations that are characteristic of 

 the transpiration rate when conditions are as constant as possible 

 and for comparison in the second column are six readings from a 

 length of stem under pressure. In the other examples averages 

 only of a series of readings are given. The pressure required to 

 produce the rate approximating that of transpiration is found in the 

 third column and in final division are the dimensions in cm. of the 

 stems subjected to the pressure. In the majority of these and 

 subsequent experiments filtered tap water was used and fresh cut- 

 tings of the ends of the stems were made in setting up the experi- 

 ments if any considerable lapse of time intervened sufficient to 

 produce a marked variation in the readings. In the case of stems 

 under pressure this was very quickly effected, and distilled water 

 gave the same though slower declines. This difference in the 

 rates of consecutive readings shown in columns I and 2 are rather 

 suggestive of the processes employed in maintaining the current 

 in transpiring stems. Pressures result in a fairly constant rate. 

 The same is true of suction. In some of the experiments under 



