26 



INTRODUCTION TO PLANT GEOGRAPHY 



[chap. 



each of our chosen classes, we will deal with its modes of nutrition 

 and reproduction and also, in the broadest terms, with its main 

 habitats, distribution, and importance — both economically and as 

 a component of natural vegetation. 



SCHIZOPHYTA 



Bacteria : These are very simple, exceedingly minute, and 

 virtually ubiquitous organisms. Most types consist of single 

 spherical, rod-shaped, branched, or variously curved cells, with or 

 without delicate superficial thread-like processes known as flagella 

 {sing, flagellum), through the action of which they may attain a fair 



Fig. 3. — Various types of Bacteria. Among those causing serious human diseases 

 are B, anthrax; H, typhoid fever; N, cholera; Q, tuberculosis; R, leprosy; 

 S, diphtheria; T, meningitis; U, pneumonia; V, dysentery; X, tetanus. (Mostly 

 1000, but Q and V considerably more magnified.) 



degree of motility in liquid media. In other types the cells may 

 adhere together in small groups or chains, or remain attached by 

 the ends to form regular filaments. Various types of Bacteria are 

 shown in Fig. 3, most of them being magnified about 1,000 times. 

 Bacteria are found in vast numbers almost everywhere : in soils 

 and the atmosphere, in fresh and salt waters, and in many of the 

 most unlikely and unsavoury ' habitats '. Normal soils with a fair 

 percentage of organic matter contain on the average from 2,000,000 

 to 200,000,000 Bacteria per gram, while manure and sewage may 

 contain greater numbers still. 



Bacteria multiply principally by simple cell division, sometimes 



