4TO SOURCES AND TRANSFORMATIONS OF PLANT-ENERGY 



discussions of this kind the activity of the living organism and the 

 existence of the potential differences it produces are taken for granted, 

 and even during absorption, especially when preceded by digestion, 

 activities may be involved which are directly due to the living plant. 



THE MOVEMENTS OF WATER. The loss of water by transpiration, or any 

 production of osmotic substances in a cell capable of further distension, will 

 tend to draw water to the region affected from surrounding parts richer in 

 water. This action is capable of exerting suction over a greater or less dis- 

 tance, according to whether the suction is exerted upon vessels filled with 

 water or containing chains of water-columns and air-bubbles. In the former 

 case the resistance to flow is directly proportional to the viscosity of the 

 liquid, to the internal surface of the tube through which flow occurs, 

 and to the velocity of flow. In the case of circular tubes with smooth 

 walls the volume passing is greater than with any other shape of bore, 

 and the rate of flow under equal pressures in such tubes is inversely pro- 

 portional to the square of the radius and the length of the tube. The 

 total resistance due to the viscosity of the water flowing through the vessels 

 is less than the height of the tree when the vessels are filled with water, but 

 when they contain alternating columns of water and air another resistance 

 is introduced which is due to the adhesion of the surface-tension films at 

 the ends of the air-bubbles to the inward projections or perforate par- 

 titions where the segments of the vessel join. This resistance is inversely 

 proportional to the diameters of the vessels or pores, and to the difference 

 in convexity between the ends of the bubbles, and it is usually sufficient 

 to produce a total resistance equivalent to a head of water many times 

 the height of the tree. 



Ewart 1 has, in fact, calculated that the total resistance to an average 

 rate of flow in the trunks of the tallest trees may be equivalent to pressures 

 of as much as 100 atmospheres, suction-pressures which are not only 

 incapable of being generated by transpiration and osmotic action in the 

 leaves, but which also cannot be transmitted through the wood-vessels 

 to the roots. The maximal strain which a water-column free from air- 

 bubbles is able to withstand appears to be about five atmospheres, and 

 in the presence of air the greatest negative pressure produced in the wood- 

 vessels is usually not more than half an atmosphere. 



Hence it appears that a continuous adjustment equivalent to a stair-case 

 pumping action must go on in the trunks of tall transpiring trees, and Ewart l 



contents from one cell to another through 3,000 threads of rV/* diameter, and the surface-tension 

 pressure exerted at the end of the thread, if in air, is as much as 34 atmospheres. In 50 cm. 

 length of the cribral system of Cucurbita, however, a pressure of only an atmosphere would 

 suffice to produce an approximate rate of flow of 5 mm. per minute. See Ewart, On Protoplasmic 

 Streaming in Plants, 1903, pp. 29-30.] 



1 Ewart, On the Ascent of Water in Trees, Phil. Trans., 1905, p. 15 of reprint 



