THE THALLOPHYTA 



285 



contains hundreds of types which are of the utmost importance to 

 man and which form the subject matter of various special 

 sciences, notably bacteriology and plant pathology. 



In the general morphology of both their vegetative and their 

 reproductive structures the fungi rather clearly parallel the 

 algae, ranging from strictly unicellular types in the bacteria, 

 through filamentous forms, to those which have a large and 

 rather complex plant body; and displaying various types of 



Fig. 162.- — Bacteria of various types. A, motile individuals of Bacillus 

 suhtilis. B, non-motile individuals of Bacillus subtilis. C, tetanus cocci. D, 

 erysipelas cocci. E, pus cocci. F, Spirillum undula. G, gelatinous or zodglaea 

 condition of a mass of bacteria. H, cholera vibrios. 



sexual reproduction, both isogamous and heterogamous. In all 

 fungi above the bacteria, the plant "body is composed of one or 

 more filaments, each of which is known as a hypha (plural 

 hyphae). The whole mass of hyphae, which are often tangled or 

 matted together, is called the mycelium. Special absorbing 

 organs or haustoria (singular haustorium) are usually developed, 

 through which the plant draws its food from the material or sub- 

 stratum on which it grows. 



The series may best be divided into four classes : The Bacteria, 

 the Alga-hke Fungi, the Sac Fungi and the Basidia Fungi. 



Bacteria. — These plants differ from other fungi in being strictly 

 unicellular, although they may sometimes form loose colonies. 

 The cells are usually very small, ranging from about 0.025 mm. to 

 as low as 0.0005 mm. in length, so that bacteria are the smallest of 

 living things. Internally, these cells seem to be almost structure- 

 less. There is some evidence, however, that nuclear material and 

 perhaps a vague nuclear body may be present, but the very minute 



