6 SPIROGYRA. [CH. I 



Fermentation. 



In the process of fermentation, sugar is broken up into 

 C0 2 and alcohol : the bubbles of gas become entangled in 

 the sugary fluid and give rise to the scum on the surface 

 so characteristic of fermenting fluids. Small quantities of 

 glycerine and of succinic acid are also produced. 



It is easy to show that fermentation depends on the 

 life of the yeast-cell, for the process can be stopped by 

 boiling (and therefore killing) the plant. Into the difficult 

 question of the nature of fermentation and its relation to 

 respiration and to the source of energy generally I do not 

 propose to enter. 



Spirogyra. 



Spirogyra is a fresh-water weed, a representative of the 

 Algae, the great tribe to which the sea-weeds also belong. 

 It occurs in slimy tufts of delicate bright green threads. 

 Each thread is a Spirogyra plant which, although more 

 elaborate in structure than yeast, is yet of a very 

 simple construction, consisting as it does of a row of 

 cells united end to end. Each cell is precisely like its 

 neighbours, there is no division of labour, each cell being 

 responsible for its own nutrition, each growing indepen- 

 dently of the others, and each being capable of taking the 

 same share in reproduction. When one of the constituent 

 cells of Spirogyra has grown to a certain length it becomes 

 partitioned into two cells by the growth of a new 

 transverse wall. This process is called cell-division, and it 

 will appear later that it is of paramount importance in 

 the development of plants generally. It is important to 



