SCHIZOPHYTA 



453 



logically similar to that carried out by the cells of the digestive tract in the 

 mycorrhizic Orchidaceae, described in Chapter XII. p. 231. But there is also 

 another line of defence which forms the foundation of the serum-treatment 

 now so widely applied. It is based on the fact that in certain instances, when 



Us 



a. 



00&5 



i'J 



% 



t^s 



^> 



\ 



r 



Fig. 349. 



b, erysipelas cocci ; c, gonorrhoea cocci ; d, splenic 



Pathogenic Bacteria, a, Pus cocci , 

 fever bacilli ; f, diphtheria bacilli ; g, tubercle bacilli ; h, typhoid bacilli ; i, colon bacilli ; 

 k, cholera bacilli. (From A. Fischer, Vorl. u Bacterien. x about 1500.) 



a bacterial toxine is introduced into the circulation of an animal in suitable 

 amounts, there is developed in the blood-serum of the animal a substance 

 which has the property of neutralising the toxine, and is called an antitoxine. 

 Various specific antitoxic sera are thus prepared, and are used either as 

 preventive or as curative agents. This is not the place to discuss the pheno- 

 mena of immunity, but it may be stated that immunity is of two main 

 types : — it is directed either against the growth of the bacteria, on the one 

 hand ; or against the action of their toxines, on the other. 



During recent years a number of important anti-bacterial drugs have been 

 synthesised and given wide clinical application. The discovery of the non- 

 toxic, anti-bacterial substance Penicillin, produced during the growth of the 

 green mould Penicillium notation and related species on suitable media, marks the 

 beginning of a new era in the treatment of wounds and diseases, Staphylococci, 

 Streptococci and Pneumococci being among the organisms most readily affected. 



The invasion of plant-tissues by Bacteria does not appear to be so common 



as of animal tissues. The reason probably is that the cell-walls prove an 



obstacle to ready infection : but a good example is seen in the Crown Call, 



which occurs on various cultivated plants, and especially on fruit trees. It 



is prominently seen on the Paris Daisy (Chrysanthemum frutescens), on which 



it is very destructive to nursery stock, being highly contagious. The galls 



are formed just under the ground on the collar or root, and grow rapidly to 



a large size, decaying at the end of the season, but forming new galls in the 



following season round the edge of the old wound, and so on. The causal 



organism is Bacterium tumefaciens, which has been isolated, and gives like 



characters both in America and in Britain. All infected stock should be 



burned, and quicklime worked into the tainted soil. 



It will be gathered from the examples mentioned above that the various 

 modes of life of Bacteria, however peculiar, are such as to rank with the 

 activities of the Fungi, and that they may be compared with those seen in 

 other representatives of the Vegetable Kingdom. 



