162 THE INDIVIDUAL ORGANISM 



blue gum — trees which attain heights of about 300 feet and carry water 

 to their topmost twigs. 



We have, then, to seek a mechanism capable not only of holding up a 

 column of water to a maximum height of about 300 feet but also of driving 

 or pulling many gallons per day to this height. The water flows through 

 ducts of extremely small diameter, the walls of which are wettable; 

 capillary attraction could therefore account for the rise of water to a 

 height of perhaps 5 feet in the smaller tracheids, and about 2 to 3 inches 

 in the largest vessels. Atmospheric pressure could cause a rise of only 33 

 feet, even supposing the pressure in the ducts were reduced to zero by 

 transpiration from the cells of the leaf. The osmotic root pressures de- 

 veloped by some plants are considerable, but such pressures would have 

 to be enormously greater than they are to drive water to the top of a tall 

 tree, even with the aid of capillarity and atmospheric pressure. Further- 

 more, root pressures are highest in spring, when rise of water is slow, and 

 fall to low or even negative levels when the leaves are developed and rise 

 of water is rapid. Doubtless all of the factors mentioned aid in the process, 

 though they cannot themselves account for its entirety. 



The principal cause of the upward movement of water in the plant 

 seems to be transpiration pull. This is an actual pulling force exerted from 

 above and transmitted along the water columns in the tracheids and 

 vessels — not in the way that air is "pulled" into the lungs, rushing 

 into a region of lowered pressure, but in the way that a wire transmits 

 a pull through its tensile strength. 



It has been found that water enclosed in a rigid tube of small diameter 

 will transmit a considerable tension through cohesion of the water mole- 

 cules, provided the material of the tube wall is wettable and there are 

 no air bubbles present to break the column. Confined by the walls of 

 the tube, the fluid is unable to change its shape and behaves in some 

 respects like a solid. Under these circumstances a force in excess of 200 

 times atmospheric pressure is required to rupture the water column. The 

 water occupying the xylem channels seems to fulfill these conditions, 

 adhering to the rigid walls of the vessels and tracheids; it is, therefore, 

 able to transmit any pull exerted on the water threads from above. The 

 cross walls present in the xylem channels do not interfere with the prac- 

 tical continuity of the water columns, since they are perforated as well as 

 saturated with water. Experiments have shown that (1) the osmotic 

 concentrations in the upper part of a plant are more than adequate to 

 pull the water to the top, being equivalent to 10 to 20 times atmospheric 

 pressure; (2) water can be pulled up through the xylem as a cohesive 

 column ; and (3) air bubbles that occasionally enter tracheids and vessels 

 are kept from spreading into other units by the wet cell walls. 



As is explained in the next chapter, water is being continually lost by 



