Transpiration and the Ascent of Sap. 25 



sap-lifting forces, it appears that if these chains were present greater 

 forces to move the transpiration stream would be needed. This 

 becomes clearer Vvith a numerical example. A pressure of one 

 atmosphere can raise a continuous column of water about 10 ra high. 

 Suppose this column to be replaced by a J am in 's chain of the same 

 height, composed of alternate columns having the same dimensions as 

 those Schwenden er records for various trees, viz. each pair con- 

 sisting of water and air to be 0-50 mm long. To start each of these 

 into motion requires a pressure, according to Schwenden er of 

 6 mm head of water. Therefore to start the whole column upwards 

 would require a hydrostatic head = 10 X 1000 X 2 X 6 mm = 

 12 atmospheres. 



But we need have no uneasiness on this account, for the high 

 permeability of wet wood for water and the restraint it opposes to 

 the passage and enlargement of bubbles render a true Jamin's 

 chain such as is formed in glass tubes impossible in the conducting 

 tissues. For the water, which in a true Jamin's chain is prevented 

 from passing the bubbles, is free, by escaping through the walls to 

 pass unopposed, while the bubbles are compelled to be stationary. 



In quite another way the Jamin's chain might be thought 

 available. It is evident that the compound column formed of an 

 alternate series of air and water indices has a smaller average 

 density than a continuous water-column, depending upon the percentage 

 of air included. Supposing the air-bubbles to occupy slightly more 

 than half a tube, then the Jamin's chain in it would have but half 

 the density of a continuous column of water. Hence it might appear 

 that a J a m i n 's chain of these proportions might be raised to double 

 the height to which the same pressure would raise a continuous water 

 column. And with a higher percentage of air greater heights could 

 be obtained. 



But this system evidently cannot be transferred to the stems of 

 trees. There, as we have seen, the conditions are such that the air 

 bubbles and water do not form a chain, or a number of chains, in 

 which all — air and water — move as a whole upwards; but, while 

 the bubbles are held stationary, the water is free to move under the 

 action of any pressure applied to it, and consequently the pressure at 

 the base of a tree must support at least the full hydrostatic pressure 

 of the water in the tree. 



Putting aside the increased difficulties introduced by the Jamin's 

 chain, it is hard to form any conception as to how the ascent of 

 water could be effected in the form of a Jamin's chain moving as 

 a whole upwards. Evaporation abstracts the water ; but what becomes 

 of the air-bubbles? The resistance opposed by the moist cells walls 

 to undissoloved gas must imprison them in the upper terminations of 



