Transpiration and the Ascent of Sap. 



49 



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 

 bordered pit. The membrane and torus of each bordered pit in the 

 conducting tracheae is able to take up three positions — a median 

 position, symmetrically dividing each domed chamber of the pit from 

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

 is naturally assumed by the more or less tightly 

 stretched membrane 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. P a p p e n h e i ra ^) found 

 that an immense 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 mem- 

 brane round the torus is very permeable to 

 water and, consequently, water moving at a 

 moderate 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 membrane will not tend 

 to displace it. Hence it is that the tensile 

 transpiration current, passing from one trachea 

 to another through the bordered pits, experien- 

 ces only the minimal resistance of the porous and thin membrane. 

 But the very delicacy and porosity of the membrane render it 

 unsuitable for sustaining any severe stress, and so we find, when a 

 bubble develops in a trachea and is gradually distended by the tension 

 in the liquid, or by a difference of gas pressure, till it fills the 

 trachea, the membranes of the pits in the walls of the trachea become 

 aspirated away from the bubble, and the membrane is supported by 

 the dome, while the torus lies over the perforation in the latter like 



Fig. 5. Diagram to show 

 the position of the pit- 

 membranes when the ten- 

 sile transpiration current 

 is passing- through them, 

 and when a bubble is 

 developed on one side. 



*) K. Pap pen he im. Zur Frage der Yerschlussfähigkeit der Hoftüpfel im 

 Splintholze der Coniferen. Ber. d. Deutsch. Bot. Gesell., VII, 1889, pp. 2 et seq. 

 Progressas rei botanicae III. ^ 



