228 STRUCTURAL BOTANY 



No known physical causes, however, will explain the 

 whole phenomenon. Atmospheric pressure will only 

 raise water to a height of about 33 feet. We know 

 that some trees are 400 or more feet high, and yet 

 their topmost branches suffer no lack of water. 

 Capillary action, by which water rises in narrow tubes, 

 will not account for the rise of sap in the vessels and 

 tracheides, for these natural tubes are not nearly 

 narrow enough to raise water to anything like the 

 height required. Thus, in a tube 5^ inch in diameter, 

 which would be rather small for a wood- vessel, water will 

 only rise by capillarity to a height of about two feet. 



A very tempting explanation, which many botanists 

 have adopted, is to suppose that the living cells of the 

 wood take part in the work, absorbing water from the 

 vessels by osmosis, and pumping it out again under 

 pressure, just as we know takes place in the case of 

 the living cells of the root. 



As, however, it has been proved that the sap will 

 continue to rise in stems more than 33 feet high, 

 which have been completely killed, we are not at 

 liberty to accept this explanation. 



All we can say at present is, that the first ascent of 

 sap in spring is due to the root-pressure, and its 

 more rapid movement after the leaves have expanded, 

 to transpiration. The fact that a column of water 

 will bear a considerable tensile strain, has lately been 

 called in to explain how the sap may be pulled up in 

 consequence of transpiration. 



V. Respiration. We will now leave the subject of 

 nutrition, and say something about a chemical process 



