THE MATERIAL OUTGO OF PLANTS 351 



which the tallest trees attain. Atmospheric pressure therefore is 

 utterly inadequate at best. The most that can be allowed is this: 

 by how much the difference in atmospheric pressure in the tracheae 

 and in the air tends to make it easier for water to pass through the root 

 hair and the root cortex, by so much atmospheric pressure may be said 

 to help in the entry of water. But the very fact that these differences 

 exist shows that they are not compensated by the movement of the 

 water. In fact the difference betweea inner and outer pressure seems 

 to be rather a result than a cause of water movement. 



Role of living cells. — The ultimate cause of the ascent of sap is tran- 

 spiration; but how it acts is entirely unknown. The energy employed in 

 vaporizing the water is adequate to lift it miles high; but how is it ap- 

 plied so as to keep a continuous stream rising? 



One link in the chain is the osmotic relations of the living cells of the 

 leaf; for if the leaves be killed, evaporation continues from their cells, 

 but the supply from the xylem strands is interrupted and the leaf dries 

 up promptly. 



It was also proposed, first many years ago, to ascribe the ascent of 

 water to the action of living cells along the course of the xylem strands, 

 and this theory is being advocated again to-day. One notion of their 

 action was that it is like that of relay pumps, which take water in at one 

 level and force it up to a higher level. It is difficult to conceive the 

 physics of such an operation, and there is no anatomical evidence of such 

 a mechanism, unless the cells of the pith rays are the active cells. The 

 experimental evidence as to the cooperation of live cells in the process 

 is contradictory, to say the least, and by its very nature the theory must 

 be rather vague. That the living cortex and wood parenchyma are neces- 

 sary to keep the xylem in proper condition for conduction is assumed. 



Cohesion theory. — A current theory, which also is confronted by 

 many difficulties and leaves much to be explained, is based upon the 

 fact of the cohesion of water. That seems, at first blush, like talking of 

 the strength of a rope of sand; but it is actually very difficult to break 

 a small column of water, if sidewise or shearing strains are eliminated. 

 The cohesive strength of water is variously estimated by physicists at 

 10-150 atmospheres. 



The rupture of sporangia of ferns and the anthers of flowering plants, and the 

 collapse of cells on drying, have now been shown to depend upon the cohesion of 

 water. The mechanism for spore scattering in the sporangium of a fern, for ex- 

 ample, is illuminating. It consists of thick-walled cells around the edge, the annulus 



