88 EVOLUTION OF PLANTS 



been the subject of much controversy, some investiga- 

 tors going so far as to deny that it exists in any mem- 

 bers of the group. Recent investigations, however, 

 have proved conclusively that in some of the simpler 

 forms, at least, not only are there genuine sexual organs 

 present, but the actual fertilization has been demon- 

 strated beyond any question. In these forms, of course, 

 a direct comparison can be made of the reproductive 

 organs and the structures arising as the result of fer- 

 tilization ; but where sexuality has been completely lost, 

 which appears to be the case in most of the larger forms, 

 it is often impossible to determine positively which form 

 of spores represents properly the product of fertilization 

 in those forms where fertilization occurs. Especially is 

 this the case where several sorts of spores are developed 

 in the same plant. 



Where parasitism occurs in the Mycomycetes, it often 

 attains a degree of specialization unparalleled elsewhere 

 in the vegetable kingdom, and recalls the behavior of cer- 

 tain animal parasites. This peculiarity consists in the 

 passing from one host to another, one form of spores 

 being produced upon one host, another upon the other. 

 One of the first cases of this " heteroecism " to be studied 

 was that of one of the fungi which cause the rust of 

 wheat. It was observed that the presence of barberry 

 bushes in the vicinity of wheat fields was accompanied 

 by an unusual amount of rust upon the growing grain. 

 It was finally discovered that the fungus, which in the 

 spring formed what were popularly called " cluster- 

 cups" upon the barberry leaves, was really only one 

 stage of the same fungus which later, passing from the 

 barberry to the wheat, caused the latter to rust, and 



