STRUCTURE AND FUNCTIONS 23 



upon other plants is much greater than those which 

 live upon animals. 



There are three types of parasitism : 



1. Internal, free parasites, which float or swim in 

 the liquid parts of the host. The bacteria, the 

 myxomycetes, and the Synchytria belong to this 

 type. 



2. The internal, fixed parasites, which usually form 

 mycelia within the tissues of the host and grow to the 

 surface only for the purpose of reproduction. They 

 may discolour the surface or they may cause deformities. 

 In some cases they are confined to a very small part 

 of the host, while in other cases they spread throughout 

 all or a considerable part of the host. (The Uredinales, 

 Peronosporales, etc.) 



3. The external parasites, which form mycelia on 

 the surface and penetrate only the outer cells. (The 

 Erysipheae.) 



Facultative Parasites. Some fungi have the power 

 of becoming either parasitic or saprophytic as the 

 conditions may necessitate. They are known as 

 facultative parasites, and some of them, of which certain 

 "damping off'" fungi are conspicuous examples, are 

 very troublesome in agriculture. 



Trichosphaeria sacchari, a parasitic fungus, which 

 is the cause of a disease of the sugar-cane, is thought 

 by some workers to have been originally saprophytic. 

 It is possible that the increase of injurious insects or 

 other causes so weakened the cane as to make it a 

 prey to this organism. The organism having once 

 gained a foothold on the weak cane, increased in parasitic 

 strength, and eventually became the severe pest which 

 we now recognize (see pages 52, 81, 132). 



Reproduction. The fungi do not produce seeds, 

 but small microscopic bodies which answer the same 

 purpose. The methods of formation of these spores in 

 the different groups of fungi can be classed under 

 two general methods ; then on-sexual, in which but a 

 single individual plant is concerned, and the sexual, in 



