YEAST. [CH. I 



the cell-wall separate from the protoplasm by pressure 

 applied to the cover-glass of a mounted preparation. The 

 protoplasm is squeezed out of the broken cell (just as the 

 flesh may be squeezed out of a grape-skin) and the torn 

 empty walls and crushed fragments of protoplasm remain. 



The cell-wall is a colourless membrane made of a 

 substance called cellulose. Cellulose is a compound of the 

 greatest importance in plant-physiology. It forms a large 

 proportion of the substance of plants, and is the basis of 

 many products of vegetable origin. Cotton wool, which is 

 made from the hairs on the seeds of the cotton-plant is 

 nearly pure cellulose, and the same is true of filter-paper 

 which is manufactured from vegetable cell-walls. 



In a wooden match are many thousands of cells of which 

 the walls are cellulose 1 . If such a piece of wood is dipped 

 in strong sulphuric acid or is charred by fire a mass of 

 charcoal is the result. This fact proves that cellulose 

 contains carbon, and as a matter of fact carbon makes up 

 nearly half the weight of this substance. 



Cellulose also contains hydrogen and oxygen in the 

 proportion in which they exist in water, its formula being 

 C 6 H 10 6 . This is the same formula as that of starch, an 

 important fact, as will appear later on. In spite of the 

 identity of formula the two substances have not identical 

 reactions. Starch is characterised by giving a blue or 

 purple colour with iodine. Cellulose is characterised by 

 not giving this test unless it has been previously treated 

 with acid. In the laboratory it is usual to shorten the 

 process by the use of an acid preparation of iodine. The 

 1 More accurately lignified cellulose. 



