Lecture XXV. 227 



drawn from the tracheids of the conducting tracts. The 

 water in the tracheids wets and adheres to their walls. There 

 are no bubbles in these tracheids, and consequently their watery 

 contents are in a condition to exercise their cohesion and to 

 transmit the pull, generated by evaporation, to the water in the 

 neighbouring tubes, and similarly from these to the water lower 

 down in the woody tubes of the leaf, stem and root. So the 

 water is dragged up from the roots just as a rope or a wire 

 might be. 



In a small plant like the Buttercup the tension developed to 

 raise the water in the plant is probably insignificant. But in 

 tall trees the drag must be very considerable. Not only must 

 the weight of the column of water (it may be 200-300 feet 

 in length), but also the resistance of the narrow tubes and 

 of the cross partitions must be overcome. The cohesion of 

 water containing dissolved air, or its tensile strength, has been 

 shown to be about 300 atmospheres, i.e. about 4500 Ib. to the 

 square inch, or 310 kgm. to the square cm. So that even the 

 pull required to draw up the water in high trees would not seriously 

 tax its strength. The sudden bendings of the stem, however, due 

 to wind and possibly to other causes, will at times break the water 

 columns in the tubes. However, when bubbles or ruptures are 

 formed they are confined to the tracheid or vessel in which they 

 are developed and thus they cannot spread so as to cut across 

 the whole water supply. Evidently the subdivision of the con- 

 ducting tracts into the small tracheids and vessels has this impor- 

 tant result that it confers stability on the stressed water. From 

 any other point of view the subdivision must be a great disadvan- 

 tage, interposing as it does so much resistance to the movement 

 of the current. The pits in the subdividing walls lessen this 

 resistance as the thinner spots interpose less obstruction to the 

 moving water than does the thick wall. At the same time the thin 

 membranes are not so strong to prevent the rupture spreading 

 from the element which contains it. The structure of the bordered 

 pit is a mechanism to give the necessary strength to the membrane 

 and at the same time leave its permeability unimpaired. When 

 there is water on both sides of the membrane and the membrane 

 occupies a median position, the resistance is low ; but if a 

 rupture in the water develops on one side of it, the water is 

 drawn away by the tension from that side, but adhering to the pit- 

 membrane drags the torus against the dome. In this position 

 the dome, which is thick and woody, supports the thin membrane 

 and the torus fills up the perforation in* the dome. Thus the 



15* 



