The Ascent of Water in Trees. 555 



In a cut stem, apart from the blocking at the cut surfaces, a gradual 

 diminution of conductivity occurs along its entire length after water 

 has been passed through for some time. This appears, in part at 

 least, to be due to the development of micro-organisms in the vessels, 

 but may be aided by swelling, by lessened permeability, or by other 

 changes in their walls. 



The length of the vessels in the wood of the branches examined 

 averages from 7 to 36 centimetres, the tracheides of the yew being from 

 0-2 to 0-5 of a centimetre in length. Since, however, the vessels appear 

 mainly to end at the nodes where branches arise, it is possible that 

 they may be much longer in the young wood on old bare trunks. The 

 resistance to transverse flow through saturated wood is 800 to 45,000 

 times greater than to longitudinal flow, the resistance to filtration 

 under pressure through a single partition wall being from 2 to 10 times 

 greater than that to the flow through the entire length of a vessel filled 

 with water in the wood of a crab apple. 



The total resistance to flow in the erect stems of actively transpiring 

 plants appears to correspond to a head of water of from 6 to 33 (shrubs 

 and small trees), or from 5 to 7 (large trees) times the height of the 

 plant. Hence in the tallest trees the total pressure required to 

 maintain active transpiration may be equivalent to as much as 

 100 atmospheres. 



No leaf could produce or maintain an osmotic suction of this intensity, 

 and in the presence of large air-bubbles in the vessels the stress 

 transmitted in them from the leaves could never be as great as an 

 atmosphere. Vines* found, for instance, that the suction force of a 

 transpiring branch was never greater than two-thirds of an atmosphere. 

 The supposition that these forces might summate is entirely erroneous. 

 On the contrary, the leaves at the base of a tree would pull water 

 down from the upper vessels and leaves, instead of up from the roots, 

 in the absence of any pumping action in the stem, and of any root- 

 pressure. 



If the air-bubbles in the vessels were exceedingly minute, they might 

 be under a small positive pressure, while the water outside was under 

 a maximal strain of five atmospheres. This would suffice to overcome 

 the resistance offered during active transpiration by 30 to 80 feet of 

 stem, hence the results obtained by Strasburger with dead stems. The 

 maximal osmotic suction exercised by the leaves, as determined by 

 comparing the osmotic pressures during active transpiration of the 

 leaves at the top and bottom of an elm 18 metres high, appears to be 

 from 2 to 3 atmospheres, and is usually less than this. At the same 

 time the total resistance to flow in the trunk of this tree would be 

 from 10 to 12 atmospheres. 



It appears, therefore, that to maintain flow, a pumping action of 

 * ' Annals of Botany,' 1896, TO!. 10, p. 438. 



