CHAPTER V 



SPECIALISATION 



The Uredinales are strictly parasitic (obligate parasites). 

 Many parasitic Fungi can live for a time saprophytically, but 

 those belonging to the present group are quite incapable of 

 such an existence. They are wholly dependent upon their 

 host. Moreover, a study of the evolution of the Uredinales 

 shows us that they have sprung from some simple beginning 

 (resembling perhaps remotely the Ustilaginales) in such a way 

 that new forms ever appeared as new hosts were evolved, and 

 advanced 'pari passu with them. The lowest forms are those 

 parasitic upon the Ferns, the highest are among those on the 

 Composita- and others of the more specialised orders. 



Each form is more or less closely adapted to its particular 

 host, but there is a wide range among them in this respect. A 

 species which can find sustenance upon hosts of more than one 

 kind is called plurivorous. One of the widest is Puccinia 

 Malvacearwm, confined, indeed, to the Mallow family, but 

 appearing to spread to nearly every genus of the group Malveae 

 of that family. It has been experimentally shown that it can 

 be transferred from Malva to Althaea and vice versa, and obser- 

 vations on its occurrence in nature imply that it can pass 

 equally to other genera (see under that species). Or a species 

 n i.i y be found only on part of a subfamily, as P. Arenariae on 

 many genera of the Alsineae. Others are confined to a single 

 genus, but appear to be equally at home on almost any species 

 belonging to it, as P. Viulae. Still others are restricted so far 

 as we know to a siugle species, as P. Buxi, and various species 



