Miscellaneous Intelligence. 469 



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-organ- 

 isms 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 exam- 

 ined averages from 7 to 36 centimeters, the tracheides of the 

 yew being from 0-2 to 0*5 of a centimeter 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 trans- 

 piring 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 equiva- 

 lent 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 sum mate 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 transpira- 

 tion 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 meters 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 some kind or other must be exercised in the wood, for which 

 the presence of active living cells is essential. In support of 



* Annals of Botany, 1896, vol. x, p. 438. 



