CHEMiaTRT AND PHTSIOS OF PLANTS. 75 



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 cellulose molecules for the layers of 

 water which surround them. 



145. The rapidity of the upward movement of water 

 varies greatly in difiEerent plants and under different con- 

 ditions. In a silver-poplar a rate of 23 cm. (9 in.) an hour 

 has been observed; in a cherry-laurel 101 cm. (40 in.); and 

 in a sunflower 22 metres (72 feet). 



Additional Notes on the movement of water in the plant. 



I. Boot-Pressure. — 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 con- 

 siderable height. Hales more than a hundred and fifty years ago 

 observed a pressure upon a mercurial gauge equal to 11 metres 

 (36 5 ft.) of water when attached to the root of a vine (Vitis). Clark 

 (1873), in a similar manner, found the pressure from a root of a birch 

 (Betula lutea) to be equal to 25.8 metres (84.7 ft.) of water. This root- 

 pressure appears to be greatest when the evaporation from the leaves 

 is least; in fact, if the experiment is made while evaporation is very 

 active, there is always for a while a considerable absorption of water 

 by the cut end of the root, due probably to the fact that the cell-walls 

 had been to a certain extent robbed of their water by the evaporation 

 from above. Root pressure is probably a purely physical phenom- 

 enon, due to a kind of endosmotic action taking place in the root- 

 cells. 



II. The Flow of Water (sap) from the stems and branches of certain 

 trees, notably from the sugar-maple, appears to be due to the quick 

 alternate expansion and contraction of the air and other gases in the 

 tissues from the quick changes of temperature. The water is forced 

 out of openings in the stem when the temperature suddenly rises; 

 when the temperature suddenly falls, as at night, there is a suction 

 of water or air into the stem. When the temperature is nearly uni- 

 form, whether in winter or summer, there is no flow of sap. 



III. No Circulation of Sap — While there is an upward movement 

 of the water in plants because of the evaporation from the leaves, 

 there is no downward movement as has been popularly supposed. 

 The "circulation of the sap," in the sense that there is an upward 

 stream in one portion of the plant and a corresponding downward 



