CHAPTER VI. 



PLANT PARASITES. 



THE plant parasites of man belong among the simplest of living or- 

 ganisms. Three distinct groups are of frequent occurrence in or upon 

 the body. These are : 



1. Bacteria, or fission fungi (Schizomycetes). 



2. Yeasts, or yeast fungi, or sprouting fungi (Blastomycetes). 



3. Moulds, or mould fungi (Hyphomycetes). 



The first group, the bacteria, is of the greatest significance, because 

 it contains organisms which are very frequently the excitants of serious 



I. Bacteria. 



MORPHOLOGY, PHYSIOLOGY, AND DISTRIBUTION. 



Bacteria are minute unicellular plants devoid of chlorophyll, multi- 

 plying by transverse division and in some cases preserving the species 

 by the formation of spores. 



The colorless, sometimes granular, protoplasm is enclosed by a mem- 

 brane, and some forms are surrounded by a transparent capsule. Not 

 infrequently parts of the protoplasm appear less 

 dense than the rest, as if from vacuolation, and a 

 ^ ew t )servers have claimed to demonstrate in cer- 

 tain forms a nuclear structure. But owing to their 

 minuteness, studies of the structure of the proto- 

 plasm of bacteria have thus far led to but meagre 

 results. 



Many of the bacteria, especially bacilli and 

 spirilla, less frequently the cocci, have hair-like 

 ^FIG. TI.-TYPICAL FORMS processes called flagella which are apparently or- 

 CILLI, AND SPIRILLA. ' gans of locomotion (Fig. 72). These may be single 

 or in tufts ; may be at one or at both ends or over 

 the general surface. Their number and distribution seem sometimes to 

 be characteristic of special forms. 



Forms of Bacteria. The various forms of bacteria may be grouped 

 into three classes (Fig. 71). 



1. Spheroidal bacteria. Cocci or micrococci (singular, coccus, 2 micro- 

 coccus). 



1 The term micro-organism includes all of these forms of minute and lowly plants. 

 They are sometimes spoken of collectively as germs or microbes. 

 'Pronounced kok'-us, plural kok'-si. 



