General PJiysiology of tJie Plant. 



199 



FIG. 104. -DIALYSIS 

 (Edmoiv's.) 



solutions, both of which are crystallisable, a current will be 



set up through the membrane between them. .This passage 



is known as osmosis. The current from the outer to the 



inner vessel in the experiment is known as an endosmotic 



current ; that from the inner to the outer is termed an 



exosmotic current. Almost all crystallisable substances, 



which can wet the membrane, are capable 



of undergoing osmosis ; non-crystallisable 



substances, as a rule, are not. Further, 



many substances pass much more rapidly 



through organic membranes than others, 



so that there may be a very rapid endos- 



mosis accompanied by a very slow exos- 



mosis, or 77?* versa. Moreover, the rate 



is also modified by the chemical nature 



of the substances inside or outside the 



membrane. 



A root-hair is a vessel, a very small 

 one it is true, containing in its cell-sap 

 certain crystallisable and certain non- 

 crystallisable substances. These are se- 

 parated from the soil by two vegetal 

 membranes, the cell-wall and the proto- 

 plasm. The soil contains various crys- 

 tallisable salts in solution in water, or 

 capable of being made so by the action 

 of various acids &c. Here, then, we 

 have all the conditions for osmosis. 

 There will be a considerable endosmosis 

 and little exosmosis, for the substances 

 in the cell being in great part protoplasmic in their nature, 

 and therefore non-crystallisable, will have little tendency to 

 pass outwards into the soil, whilst the crystallisable solutions 

 in the soil will tend to pass inwards through the cell-wall 

 and protoplasm into the cell. It must be remembered, 

 however, that the protoplasm itself acts in many cases 



n, level of water in outer 

 vessel ; /', inner ves- 

 sel containing salt so- 

 lution ; r, level of salt 

 solution after experi- 

 ment. 



