Lecture III. 23 



that when heated the semi-permeable protoplasm of the beet-cell 

 became permeable and let the dissolved pigment escape. Coagu- 

 lation may also be effected by chemical means, or even by the 

 presence of other colloids. There is reason to believe that 

 coagulation is sometimes due to the neutralisation of electric 

 charges on the colloidal particles. 



The cells we have hitherto examined belonged to great masses 

 of cells forming the tissues of higher plants. Being composed of 

 aggregates of cells, these plants are called multicellular plants. 

 More simple plants are known consisting of fewer cells : and the 

 simplest of all are those which, consisting of a single cell, are. 

 termed unicellular plants. 



To one of these we will now turn our attention. 



It is the well-known plant which is utilised in the rising of 

 bread and in the brewing of beer the Yeast plant. Supplies 

 may be easily obtained from a brewery or from any shop which 

 supplies yeast for bread-making. 



Yeast as thus obtained is a plastic buff-coloured mass. It is 

 friable and for microscopic examination a fragment the size of a 

 pea may be broken off and shaken up in a small test tube contain- 

 ing a sugar solution (say 10 or 15 per cent.). After continued 

 shaking the mass of yeast disappears and is distributed through 

 the sugar solution, forming a milky suspension. 



If a drop of this suspension is placed on a slide and covered 

 with a clean cover-glass and examined with the low power, a vast 

 crowd of very minute specks will be seen. These specks are uni- 

 form in size and have a very definite outline. Each is, in fact, 

 a single cell of yeast, and each cell constitutes a complete plant. 

 This yeast plant is known botanically as Saccharomyces cerevisea. 



By applying the high power something more of the plant's 

 structure may be made out. Even with the high power it still 

 appears very minute. When carefully measured it is found to be 

 an egg-shaped cell, just attaining the i/ioo mm. in its longest 

 diameter. As the cells are very uniform in size this yeast plant 

 is very useful as a standard of microscopic measurement. 



When the focus is adjusted to the equator of one of these 

 unicellular plants it is possible to see the very sharp line which 

 limits the outside of the cell. This line represents the outer 

 surface of the cell-wall. The thickness of the cell-wall can 

 scarcely be made out without a special procedure. Within the 

 cell-wall is the protoplasm of the cell. When the optical section, 

 or focal plane, coincides with the equator of the cell one can see 

 that there is a zone of brilliant or highly refringent material 



