THE WATER IN THE PLANT. 173 



evaporation is rapid ; in a short time the leaves wither and 

 become dried up, unless the cut portion of the shoot be 

 placed in a vessel of water ; in the latter case the water will 

 pass rapidly into the shoot, and the leaves will retain their 

 normal condition. If in such an experiment a colored watery 

 solution (as of the juice of Poke berries) be used instead of 

 pure water, it will be seen that the liquid has passed more 

 abundantly through certain tracts than through others, in- 

 dicating that the tissues are not equally good as conductors 

 of watery solutions. As would readily be surmised, the 

 tissues in ordinary plants which appear to be the best con- 

 ductors are those composed of elongated wood-cells, and it is 

 doubtless through them that the greater part of the water 

 passes. Furthermore, it is probable that the movement of 

 the water is through the substance of the cell- walls, and not, 

 at least to any great extent, through the cell cavities. Ac- 

 cording to this view, the force which raises the water, in 

 some cases to the height of a hundred metres or more, is the 

 attraction of the surfaces of the crystal molecules for the 

 layers of water which surround them. 



226. — The rapidity of the upward movement of water evi- 

 dently varies directly as the rapidity of evaporation, and in- 

 versely as the area of the conducting tissue in transverse sec- 

 tion. As both these factors are variable, it is impossible to 

 give an average rate of movement. Sachs estimated the 

 rate of ascent in a branch of the Silver Poplar, from which 

 there was strong evaporation, at 23 cm. (9 in.) per hour. 

 McNab, by watering plants with a solution of lithium citrate 

 and then examining the ashes at successive points, found the 

 rate in a Cherry Laurel to be 101 cm. (40 in.) per hour. Pfit- 

 zer obtained the astonishing result of 23 metres (72 ft.) per 

 hour in the Sunflower ; there is but little doubt, however, 

 that this is entirely too high. 



(as) In addition to tlie movements of the water described above, that 

 which has been called root pressure requires a brief mention. If the 

 root of a vigorously growing plant be cut off near the surface of the 

 ground and a glass tube attached to its upper end, the water of the root 

 will be forced out, often to a considerable height. Hales* noted a pre.ssure 



* Statical Essays, p. 114. 



