16 GENERAL MORPHOLOGY AND BIOLOGY 



spores. Sometimes too the protoplasm of the filaments breaks 

 up into bacillus-like elements, which may also have the capacity 

 of originating new individuals. In the streptothrix actinomyces 

 there may appear a club-shaped swelling of the membrane at the 

 end of the filament, which has by some been looked on as an 

 organ of fructification, but which is most probably a product of 

 a degenerative change. The streptothrix group, though its 

 morphology and relationships are much disputed, may be looked 

 on as a link between the bacteria on the one hand, and the 

 lower fungi on the other. Like the latter, the streptothrix forms 

 show the felted mass of non-septate branching filaments, which 

 is usually called a mycelium. On the other hand, the breaking 

 up of the protoplasm of the streptothrix into coccus- and bacillus- 

 like forms, links it to the other bacteria. 



GENERAL BIOLOGY OF THE BACTERIA. 



There are five prime factors which must be considered in 

 the growth of bacteria, namely, food supply, moisture, relation to 

 gaseous environment, temperature, and light. 



Food Supply. The bacteria are chiefly found living on the 

 complicated organic substances which form the bodies of dead 

 plants and animals, or which are excreted by the latter while 

 they are yet alive. Seeing that, as a general rule, many bacteria 

 grow side by side, the food supply of any particular variety is, 

 relatively to it, altered by the growth of the other varieties 

 present. It is thus impossible to imitate the complexity of the 

 natural food environment of any species. The artificial media 

 used in bacteriological work may therefore be poor substitutes 

 for the natural supply. In certain cases, however, the conditions 

 under which we grow cultures may be better than the natural 

 conditions. For while one of two species of bacteria growing 

 side by side may favour the growth of the other, it may also 

 in certain cases hinder it, and, therefore, when the latter is 

 grown alone it may grow better. Most bacteria seem to 

 produce excretions which are unfavourable to their own 

 vitality, for, when a species is sown on a mass of artificial 

 food medium, it does not in the great majority of cases go on 

 growing till the food supply is exhausted, but soon ceases to 

 grow. Effete products diffuse out into the medium and prevent 

 growth. Such diffusion may be seen when the organism pro- 

 duces pigment, e.g. b. pyocyaneus growing on gelatin. In 

 supplying artificial food for bacterial growth, the general principle 

 ought to be to imitate as nearly as possible the natural surround- 



