LECTURE III. 



IN the study of the cells of the leaf-skin and of the beetroot 

 (or rhododendron petal) very important properties of the materials 

 which compose cells have come to light. The knowledge of 

 these properties explains how the substances perform various 

 functions which are assigned to them in the cell. Thus the 

 tenacity of the cell-wall supports the plastic protoplasm when 

 the semi-permeability of the latter obstructs the passage of 

 dissolved particles. So the rigidity of the cell is secured by 

 a tension member, the cell-wall, and a compression member, 

 the mutually repelling dissolved particles in the vacuole. The 

 rigidity of tissues and of many organs is explicable by the rigidity 

 of their component cells. Hence it is that, when the osmotic 

 pressure of the cell vacuoles of a herbaceous tissue or organ 

 is relieved by making the semi-permeable protoplasm permeable 

 by heat, the rigidity of the whole tissue or organ disappears. 



The dissolved substances in the vacuoles which give rise to 

 this osmotic pressure and force the protoplasm out against the 

 cell-wall are very various. Chemical tests show that they may 

 be organic and inorganic acids and their salts, also carbohydrates. 

 In addition to these bodies which diffuse and generate osmotic 

 pressure, there are also found in the vacuole other bodies which 

 have not got these characteristics. The former are said to be 

 in the crystalloidal state, while the latter are described as being 

 in the colloidal state. These names were applied because when 

 the water, throughout which the former substances are distributed, 

 is evaporated, the substances crystallise out, while under similar 

 circumstances the latter form a colloidal or glue-like mass. The 

 most important substances which are found in cell vacuoles in 

 the colloidal state are proteins, dextrins and gums. 



The difference of these two states with regard to diffusion 

 may be easily shown by experiment as follows : In this jar there 

 has been placed a small quantity of a 10 per cent, "solution" 

 of gelatine mixed with some Congo red a collodial dye. In 

 its fellow is some of the same gelatine solution to which had 

 been added some copper sulphate. In each case care was 



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