STRUCTURE AND FUNCTIONS 19 



parts of themselves. This process is briefly summarised 

 in the following diagram : 



the food of 



Animals and plants 

 not possessing chloro- 

 phyll, which disin- 

 tegrate complex 

 chemical compounds, 

 liberating energy in 

 the process. 



'HjjO 



->Salts 



Radiate energy chiefly as heat. 



When a plant or animal dies and undergoes decay, the 

 various substances are reduced and returned to the air 

 and soil, and may eventually form the food of plants. 



BACTERIA 



The processes of decay are carried on by certain 

 bacteria, aided to some extent by fungi and other 

 organisms. They are closely related to the lower fungi 

 and lower algae, and are the smallest and probably the 

 simplest forms of life known. There are an enormous 

 number of species, and they are more widely distributed 

 and live under more diversified conditions than the 

 members of any other group of plants ; in varying 

 depths of water, in air, in soil, in and on both dead 

 and living plants and animals, and in extremes of 

 temperature and desiccation. They live either singly 

 or in colonies, and can be classed in three general 

 groups with reference to form ; (a) the coccus forms 

 which are spherical, the bacterium or bacillus forms 

 which are oblong or rod - like, and the spirillum 

 forms which are curved or twisted. Some are ciliated 



