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 “ hetercecism ” 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 finaily 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 
