174 BOTANY. 



upon a mercurial gauge equal to 11 metres (36.5 ft.) of water when at- 

 tached to the root of a vine ( Vitis). Clark,* in a similar manner, found 

 the pressure from a root of the 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 experi- 

 ment is made while transpiration 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 ex- 

 tent robbed of their water by the evaporation from above. Root pres- 

 sure is probably a purely physical phenomenon, due to a kind of en- 

 dosmotic action taking place in the root- cells. 



(b) The flow of water (sap) from the stems aud 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 uniform, whether 

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



II. As TO SOLUTIONS. 



227. The water in the plant holds in solution several 

 substances, so that it is not water alone, but in reality a 

 complex solution. Some of the substances in solution are 

 solids, as the inorganic salts taken up from the soil or water, 

 while others are gaseous, as the air and carbon dioxide taken 

 up in the water by the roots, or absorbed by the leaves and 

 there entering into solution in the water. The final use of 

 these solutions will be spoken of further on ; here it is only 

 necessary to point out some of the more important general 

 facts as to solution and diffusion : 



1st. When a substance has entered into solution it still 

 exists as that substance, and the water in which it is dis- 

 solved is in one sense pure. This is readily shown by driving 

 off the water by heat, when the dissolved substance is again 

 obtained in its original solid state. 



2d. As soon as solution begins the process of diffusion 



* In 1873, recorded in the Twenty-first Report of the Secretary of 

 the Massachusetts State Board of Agriculture. See also further re- 

 sults by the same observer in the Twenty -second Report. 



