ABSORPTION AND TRANSMISSION OP WATER 109 



sap from the adjacent tracheae and passing it on upwards. 

 But if sap has already passed through a set of these active 

 cells in a manner similar to that described in connection 

 with water pores, its concentration cannot be lower than 

 that of the cell sap of these cells when they are stretched to 

 their utmost ; indeed, from the loss of pure water to other 

 cells along its path its concentration is apt to be even 

 higher. If this same sap, now in the trachea3, is to enter 

 another set of such active cells and be pressed still higher 

 up, the sap of the latter must necessarily possess a higher 

 osmotic concentration than the fluid to be absorbed; and 

 after it has been pressed out of them into tracheae still 

 farther up the stem, it must have gained in concentration. 

 Thus any such explanation of the rise of sap in stems 

 involves a gradually increasing concentration of the sap as it 

 passes upward. There seems to be no experimental evidence 

 of this as a fact. It is true that the sap in leaves is more 

 concentrated than that in the stem, but there seem to have 

 been put on record no observations of a gradual increase in 

 concentration toward the summit of a tree. Evaporation 

 from the leaves would account for the observed fact. 



The above presentation will stand for various hypotheses 

 which have been proposed in this regard, all involving some 

 sort of a periodic variation in permeability. Godlewski and 

 Janse have attempted to locate the active cells in the cortex 

 or medullary rays of woody stems, but these attempts have 

 apparently failed. In fact, the whole idea that the ascent of 

 sap in tall stems has any necessary dependence upon the 

 presence of living cells may be doubted very much on the 

 following experimental grounds: 



In his study of bleeding Wieler * came to the conclusion 

 that this process, while it is probably a general property of 



1A. WIELER, "Das Bluten der Pflanzen," Cohns Beitrtige, Vol. VI (1893), pp. 

 1-211. 



