124 



CIRULAT1ON OP THE SAP. 



2. ABSORPTION AND CIRCULATION OF FLUIDS. 



253. While the leaves and other aerial organs of plants have the 

 power of absorbing fluids, it is chiefly in the roots that this process takes 

 place. The cells of the spongioles or fibrils of the roots, are covered 

 by a very delicate membrane (*f[ 120), which allows the imbibition of 

 fluids to proceed rapidly ; and as additions are made to their extremi- 

 ties, they are constantly placed in circumstances favourable for the re- 

 ception of fresh nutriment. The nutritive materials in the soil, partly 

 derived from the decomposition of organic and inorganic materials, 

 and partly from the atmosphere, are supplied to the roots in a state of 

 solution; and as the substances in the cells of plants are usually denser 

 than the external fluid matters, a process of endosmose takes place by 

 which the latter pass through the cell-membrane in large quantities, 

 while a small portion of the former is given off or excreted by exosmose. 

 These movements have already been alluded to as taking place be- 

 tween fluids of different densities, when separated by an animal or 

 vegetable membrane (^[ 27). They are referred by some to electrical 

 agency, and they perform an important part in the motions of vege- 

 table fluids. 



254. Endosmose and Exosmose, then, are the names given to the 

 phenomena of mixture through a membrane accompanied with change 



of volume. The former being given when the volume 

 increases by an in-going strong current, the latter when 

 the volume diminishes by an out- going weak current. 

 In most cases, but not all, the dense fluid increases. 

 The rapidity of the mixture depends on the position 

 which the denser fluid occupies being quicker when it 

 is uppermost. In fig. 222 is represented the mode 

 of showing endosmose by means of a bladder full 

 of syrup, which is attached to the end of a tube and 

 immersed in water. In this case the water passes 

 rapidly into the bladder by endosmose, so that the fluid 

 rises in the tube, while a portion of the thicker fluid 

 passes out by exosmose. The force of this endosmose 

 may be measured by a graduated tube, as ha the figure, 

 or by a tube with a double curvature, as fig. 224, the 

 lower part of which is filled with mercury. In the 

 latter case, the mercury is pushed upwards into a 

 graduated tube, and thus an endosmosmeter (/AETJ ov, a 

 measure), or measure of the force of endosmose, is formed. 



255. Dutrochet found that with a membrane of 40 millimetres* in 



* A millimetre is about l-25th of an English inch. 



Fig. 222. Instrument to show Endosmose and Exosmose, consisting of a bladder containing 

 syrup attached to a tube, and plunged in a vessel of water. The inward motion of the water 

 (endosmose) exceeds the outward movement of the syrup (exosmose). 



