98 TRANSPIRATION AND ASCENT OF SAP ch. 



It is interesting to find that we often have indications 

 that the unsupported 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 between 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 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. Pappenheim 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 membrane 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 



