210 GENERAL BIOLOGY 



The relation of the yeasts or blastomycetes to the bacteria and the 

 molds is shown in the following diagram : 



Pathogenic Protophytes. 



(These are all Chlorophyl-less plants.) 



Schizomycetes ( ) 



(bacteria) 



Hyphomycetes ( 

 (molds) 



Blastomycetes ( 

 Saccharomycetes ( 



or 



Ascomycetes ( 

 (yeasts.) 



Oidia ( 



(transition form) 

 Oidium. 



Blastomyces. 



A living organism may be injured either mechanically or chemically. 



Mechanical or physical injury may be the result of violence, pres- 

 sure, heat (burning), cold (freezing), light rays, and electricity. 



Chemical injury results from poisons (toxins), whether by auto- 

 intoxication, or toxins produced by animal or vegetable parasites. 



It is readily understood that a parasite may cause either mechanical 

 or chemical injury to its host: the former by propagating so rapidly 

 that it causes a physical obstruction, as is the case with the Tubercular 

 bacillus ; and the latter by a definite poison that the foreign organism 

 produces. The poison, or the mere mechanical effect, may in turn cause 

 some out-of-the-ordinary conditions, as for example when it furnishes a 

 stimulus to overgrowth on some part of the host, as galls on trees (a 

 growth caused by the stimulus of aphids), or tumors in human beings. 

 These latter are merely overgrown healthy cells. 



These overgrown healthy cells may press against neighboring 

 blood vessels, obstructing the blood-flow and thus causing the death of 

 the tissue which fails to receive its required amount of blood. 



It may be stated in this connection that there are some hosts which 

 are not affected at all, or but little, by the poisons parasites produce, al- 

 though the parasite itself does many such hosts a great deal of damage. 



The subject of toxins is among those of which no great knowledge 

 has yet been obtained. They are of great interest and importance as can 

 readily be judged from what has been said. 



Probably, if toxins and enzymes are thought of here in relation to 



