938 MINNESOTA BOTANICAL STUDIES. 



In many instances the host is destroyed without any prelim- 

 inary morphological changes. The parasite simply enters the 

 cells and destroys them by assimilating the plasmic contents. 

 This form of symbiosis Tubeuf designates as Perniciasm. In 

 other instances, also belonging to perniciasm, there are slight 

 secondary changes before death takes place; resulting in rudi- 

 mentary galls or mere swellings. 



In other instances death is the result of ferments and ptom- 

 aines generated by the parasite, as in various diseases of ani- 

 mals as well as of plants Some parasites dissolve the cell- 

 wells of the? host, while others simply lie in contact with the 

 cells and absorb the contents by osmotic action. In a great 

 number of instances hypertrophies and abnormalities in growth 

 are induced (galls, hypertrophied fruits and leaves; enlarge- 

 ments in animal tissues). Again, atrophy, or a total check in 

 development, may occur as the result of parasitism. 



With some parasites the host adaptation has become highly 

 specialized. In the phenomena known as heteroecism the suc- 

 cessive generations of the parasite develop upon different host- 

 plants. For example, Puccinia graminis develops its aecidio- 

 spores upon Berberis vulgaris, while its teleutospores are devel- 

 oped upon some of the grasses, as wheat or oats. Most para- 

 sites do, however, not have successive autogenetic genera- 

 tions. Many are limited to one host- species, or even to definite 

 tissues or organs 



One organism may enter into different forms of symbiosis. 

 For example, the bacillus of typhoid fever may enter into an 

 accidental (perhaps contingent) s} 7 mbiosis with the oyster, 

 while with man it forms an antagonistic symbiosis. The bac- 

 illus of Asiatic cholera, likewise, may live in and upon various 

 animals without any injurious effects, but as soon as it finds its 

 way to the intestinal canal of man it acts as a true parasite. 

 The distinction into facultative and obligative parasitism de- 

 pends upon the ability that some organisms have of living as 

 parasites and saprophytes, while others are alsolutely depen- 

 dent for their existence upon association with the host. 



The most common parasites are the fungi. The Schizomy- 

 cetes form antagonistic symbioses, preferably with animals. 

 The higher fungi predominate upon vegetable tissue. Many 

 diseases of animals are also due to the higher fungi. Algae 

 occur parasitically in and upon plants and animals.- Many of 

 the Chlorophyceae and Cyanophyceae occur as parasites upon 

 higher plants. Many of the marine algae are parasitic upon 



