Indigenous Species with their Hosts. 45 



CHAPTER XIV. 



INDIGENOUS SPECIES WITH THEIR HOSTS. 



The great majority of Australian rusts complete their life-history on one 

 and the same plant, and are thus autoecious ; but, although heteroecism, or 

 the division of the life cycle into two generations, each on different host- 

 plants, has not yet been proved for any of them, still it has been so well 

 established for several in other countries, that we may accept it for the 

 present as likely to hold good here. As to the recognised indigenous species 

 which are also heteroecious, there are only four Puccinia agropyri, P. 

 agrostidis, P. magnusiana, and P. caricis the three former on Gramineae 

 having their aecidial stage on Ranunculaceae, and the latter on Cyperaceae 

 with its aecidial stage on Urticaceae. 



If we arrange the indigenous species of rusts known in Australia under 

 the different families of their host-plants, which are further classified ac- 

 cording to their predominance, as determined by the late Baron von Mueller 

 (Table) some interesting deductions may be made; but it must always be 

 remembered that the number of known species is probably far short of those 

 actually existing. Under these circumstances our conclusions can only be 

 partial, still, even with these limitations, it will be instructive to compare the 

 predominance of the native host-plants with that of the native rusts. It 

 is found that the greatest number of rust-species occurs on the families of 

 native plants which are large in point of numbers. Thus the Leguminosae 

 with the greatest number of species have eighteen different rusts, while the 

 Compositae, which only stand fourth in the list, have seventeen. At the 

 same time it ought to be noted that future discoveries may alter this rela- 

 tion, for no less than seven species of the new genus, Uromycladium, have 

 been added to the Leguminosae within /the last few years. 



The Cyperaceae, which succeed the Compositae, have only four species, 

 while the Gramineae, which come next, have thirteen species. Then the 

 Liliaceae have seven species and the Rubiaceae five ; but on the remaining 

 families they vary from one to five. The Leguminosae and 

 Compositae have eighteen and seventeen species respectively, the Gramineae 

 coming third with thirteen species. The grasses and composites are gene- 

 rally herbaceous, quick-growing plants, with succulent leaves, and 

 the rust-fungi can most readily penetrate their tissues and secure during the 

 growing season sufficiency of food. They would also be guided in their 

 choice by the chemotactic nature of the substances contained in the host- 

 plants ; but there are so many factors which enter into the choice of a host- 

 plant by a rust that we can only mainly at present note their preferences. 

 Confining our attention now to the species of Puccinia alone and compar- 

 ing them with the numbers and distribution as given in Sydow's Mono- 

 graph, it is found that while one-fourth of all the species inhabit Com- 

 positae and one-eighth occur in Gramineae, so with the naitive Puccinias in 

 Australia more than one-seventh belong to the Compositae and one- 

 ni'nth to the Gramineae. In the Leguminosae only one species has been 

 met with, and only fifteen species are recorded altogether. 



