MOTION OF THE SAP THROUGH PLANTS. 519 



mingle; as oil or resin on the .one side, which will not mingle with watery 

 fluids that may be on the other. Link (Wiegmann's Archiv, 1841) says, 

 in reference to my view of the subject, " That as the inanimate mem- 

 branes of plants resist absorption, as we daily see, so it is plain that this 

 property was originally possessed by the living membrane." This con- 

 clusion was at least rash, for we know from chemistry that there are 

 many substances that were once held dissolved in water, but which, 

 when the moisture evaporates, will not dissolve again ; so may a mem- 

 brane which, when living, was permeable to fluid, lose that power when 

 entirely dried. But it is to be regretted that Link did not pursue his 

 investigations ; as he had daily opportunities of doing so, he might have 

 rendered important service to artificers in wood, who derive from che- 

 mistry their artificial varnishes and paints, by which they prevent the 

 entrance of water into wood. I daily see that wood, linen, paper, &c. 

 are penetrated through and through by fluids, that washed boards are 

 wet to a considerable depth, that wooden vessels standing in water are 

 penetrated by the fluid one quarter of an inch, that the boat-maker 

 reckons on a certain loss in sunken wood, because, when saturated with 

 water, it will lose all the air which, when swimming, it contained ; 

 hence, also, thick wood is longer in being saturated, because the air in 

 the cells is longer escaping : this is daily experience. By scientific 

 investigation we learn that vegetable membranes are as serviceable for 

 endosmotic experiments as animal ; that the starch in the cells of a slice 

 of potato kept for a week is coloured by iodine, as in the fresh potato ; 

 that if old dead wood, pith, cotton, &c. be observed through a microscope, 

 all the cells are filled with air, but as soon as a drop of water is dropped 

 upon them the air will be absorbed and the water fill the cells. In short, 

 the living and dead membranes show no difference : the former, because 

 naturally containing more fluid, will absorb more quickly than the latter, 

 which are entirely dry, and must be wetted before they absorb. All 

 this Link might have known, and should have known, when he wrote 

 upon the subject. 



Yet with all this we have no evident movement of the sap in the 

 plant. The watery sap in the cells is scarcely at all compressible ; the 

 cell-walls are so little elastic, that in the coherence of the entire plant 

 they appear almost as fixed ; expansion and contraction is so slight, that 

 no observation gives us any intimation of it. It is quite different in 

 animals, where, in the elasticity of the walls of the vessels and the mo- 

 tility of the contiguous soft parts, the conditions are afforded of locally 

 or generally emptying or filling them. Fluid cannot, therefore, enter a 

 cell (and consequently a plant) before room has been made for it by the 

 escape of the fluid before contained in it. As, however, all cells are 

 filled with fluid, evaporation alone can empty them. In the most im- 

 portant sense, therefore, is the motion of the sap in plants, as well as its 

 presence, quantity, and direction, entirely dependant on evaporation. 

 The greatest quantity of sap flows in the direction in which there is the 

 greatest evaporation, which is constantly to the leaves and youngest 

 parts. The motion of the sap, therefore, must be strongest where the 

 plant has most evaporating organs : it is strongest in summer, because 

 there is most evaporation ; weakest in winter, because there is least. 

 Together with evaporation is another condition (chemical change), which 

 is, however, but imperfectly explained. By the change of the sap into 

 solid or fluid compounds, the specific gravity will be generally increased ; 

 and by the diminution of volume room is made for the entrance of more 



t x 4 



