424 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1910. 



mission and stability of a tensile stream, however, these thickenings 

 are essential. And their strength, so far from being superfluous, is 

 probably often tested severely in times when the transpiration re- 

 moves large quantities of water and so develops high tensions in the 

 sap. The whole wall is not thickened uniformly because the perme- 

 ability of the thinner parts is essential. The thickenings confer on 

 the thin walls the rigiditj'- necessary to support the tensile stresses 

 in the sap. 



It is interesting to find that we often have indications that the un- 

 supported wall would not in itself have sufficient rigidity to bear the 

 crushing forces it is exposed to. These indications are particularly 

 frequent in the protoxylem.^ Here commonly, when elongation has 

 widely separated the rings and spirals, the thin part of the walls of 

 the vessels is drawn in as a constriction betw^een the spiral or annular 

 supports, and often the whole vessel is collapsed if the supports have 

 become too oblique. That this is not due to the pressure exerted by 

 the growth of the surrounding tissues follows from the fact that these 

 instances are most frequently found in leaves. 



The most perfect adaptation to secure the advantages of ease of 

 flow without seriously reducing the rigidity of the tracheae is to be 

 found in the most general of all the wall structures, viz, the bor- 

 dered pit. The membrane and torus of each bordered pit in the con- 

 ducting tracheae is able to take up three positions — a median posi- 

 tion, symmetrically dividing each domed chamber of the pit from 

 the other, and two aspirated or lateral positions. The median posi- 

 tion is naturally assumed by the more or less tightly stretched mem- 

 brane when it is not acted upon by lateral forces. In the aspirated 

 positions the membrane is deflected against one dome or the other 

 and the torus lies over and fills the opening into the dome. The 

 membranes of pits in the common wall separating two adjacent 

 tracheae filled with water naturally take up the median position. 

 Pappenheim - found that an immence rush of water through the 

 pit was needed to deflect the membrane to one side. A moderate 

 flow does not disturb it from its median position. The reason for 

 this is to be found in the fact that the membrane around the torus is 

 very permeable to water, and consequently water moving at a mod- 

 erate speed passes through it easily without displacing it. 



The normal transpiration current never possesses the velocities 

 which Pappenheim found were necessary to deflect the membrane, 

 and, of course, hydrostatic tension in the liquid on each side of the 



1 H. H. Dixon, Physics of the Transpiration Current. Notes from the Botanical 

 School, Trinity College, Dublin, No. 2, 1897, p. 14. 



2 K. Pappenheim, Zur Frage der Verschlussfiihigkeit der Hoftiipfel im Splintholze der 

 Coniferen. Ber. d. Deutsch. Bot. Gesell., vol. 7, 1SS9, pp. 2 et seq. 



