338 Curtis : The Work Performed in 



discussion, also in tables 2 and 6, the pressure was replaced by 

 attaching the stems to an air pump and recording the amount of 

 suction required to draw water through the stem at the transpira- 

 tion rate. The results are the same in either case. The transpira- 

 tion current is an irregular one not indicative of a pull or pressure 

 while the distribution of water in the stem can hardly be recon- 

 ciled with the existence of such a condition. 



Another interesting feature is the wide variation in the resist- 

 ance of the stems, not alone of various plants but of the same 



species. 



The tests were made on vigorous stems, entirely free of 

 branches below, such as may be obtained from vigorous plants. 

 In such cases the wood is exceptionally regular owing to the early 

 suppression of all lateral organs. In some instances, however, 

 the cause of the variations of rate was apparent, on splitting the 

 stems, in irregularities of growth due to branching or other causes. 

 The presence of these resistances becomes more apparent if tran- 

 spiring stems are allowed to imbibe solutions of eosin when the 

 difficulty of the fluid in passing the nodes or any obstruction due 

 to irregularities in the grain of the wood is very apparent. More 

 frequently the cause of the resistance cannot be accounted for on 

 this ground and must be due to the specific pecularities of the in- 

 dividual elements rather than to any grouping of tissues that is 

 apparent to the eye. Such may be the explanation of the high 

 resistance of Pelargonium for there is sufficient carrying capacity 

 in the water conduits to render unnecessary the high pressure re- 

 quired. 



While as a rule the resistance increases with the rate there is 

 no regularity in this ratio, as will be noted in comparing the second 

 and third columns. However, it is to be noted that when a given 

 pressure produces a certain rate of flow any increase or decrease 

 of pressure causes about the same fluctuations of rate as would be 

 found in capillary tubes. Two examples will illustrate this re- 

 lation, the first number to the left being the pressure and the suc- 

 ceeding ones in the same line the rate of transmission. 



Acalypha 24 63 63 64 65 65 Taxus 58.6 6 1 61 61 62 62 63 



30 55 55 56 56 57 68.6 51 51 51 51 5 2 5 2 



36 48 48 48 49 49 » 78.6 45 45 44 45 4 6 46 

 42 42 43 43 44 44 48.6 77 77 77 77 7» 7 8 



