APPENDIX. 191 



in the intercellular space, this gas will pass into the cell by 

 endosmosis, and the same may be said of oxygen ; during the 

 assimilation of OOj and water during the day-time in the cells of 

 the mesophyll, oxygen is one of the bye-products, and this, since 

 its osmotic pressure and percentage in the cell-sap are greater 

 than those of the same gas in the intercellular spaces, will escape 

 to a certain extent by exosmosis, leading to the evolution of 

 oxygen from the surface of the leaf through the stomata. 



A given solution of either salts or a gas is said to be isotonic 

 with another solution, when no permanent interchange takes 

 place between their saline or gaseous constituents when they are 

 separated from one another by a permeable membrane. In the 

 plant cell, although for a short space of time, the condition of 

 osmotic balance may be present, this condition rarely lasts any 

 length of time, since the absorbed substances are being constantly 

 used up. The main conditions for adequate osmosis in the 

 plant-cell are : — 



a. The stability of the cell-wall and eotoplaamic membrane. 



h. The presence of dilute solutions of salts, Ac, in the soil or 

 cell-sap ; adequate dilution is necessar}', since, before molecules of 

 salt can pass through the membrane by osmosis, they must be 

 Ionised, that is to say, the atoms or atomic groups must be 

 separated from one another by the solvent. 



In the plant-cell, however, the ectoplasm exerts a regulating 

 action (selective capacity), which, to a certain extent, modifies 

 osmosis ; and the membrane of separation becomes, in a measure, 

 comparable to the semi-permeable membrane — viz., that which 

 permits of the entrance or exit of certain molecules (salts, acids), 

 but not of others (colloids). Moreover, in many instances, salts 

 may be absorbed which do not appear to have any influence 

 upon metabolism, but gain an entrance on account of the 

 smallness of their molecules, or atomic groups into which they 

 are split up. 



