RECENT ADVANCE IN THE STUDY OF FUNGI 535 



group, closely allied to the Basidiomycetes. A recent mono- 

 graph by P. and A. Sydow gives a complete account of the 

 genera and species, arranged in the order of the host-plants. 

 D. M'Alpine has since published an account of Australian rusts. 

 He notes the curious fact that only four of the indigenous 

 species are hetercecious ; all the others are autcecious, and 

 complete their life-history on one host. 



Plant diseases caused by parasitic fungi form the subject 

 of much study and research. We have not recently suffered 

 in this country from any such widespread and devastating 

 epidemics as those caused by potato disease or by hop mildew ; 

 but all cultivators know, to their cost, of the prevalence of one 

 form or another of fungal parasites that injure their crops. 

 Stem and root, leaves and fruit, all are liable to disease from 

 a large variety of fungi, and need to be guarded against attack. 

 A host of workers is engaged on this branch of plant pathology, 

 and new facts are being brought to light day by day. The life- 

 histories of these inimical fungi are being worked out, so that 

 a remedy can be applied at the precise time when it will have 

 most effect. A very valuable discovery was made by Eriksson 

 in working on rusts. He found that the morphological species 

 may include a number of biological or physiological species — 

 i.e. that there is some peculiarity in the parasite that cannot 

 be distinguished by the microscope, but which yet effectually 

 prevents it from changing its habitat. In many cases the rust 

 that grows on one form or variety of the host-plant will infect 

 no other. This has a most important bearing on the spread of 

 rust disease, and it is thus possible to avoid the cultivation 

 of the varieties most subject to attack, and to select those that 

 are proved to be immune to the prevalent disease. Marshall 

 Ward followed on the same lines, experimenting with a rust 

 that grows on brome grass, and he proved the narrow range of 

 choice of host in the parasite; he proved, however, the existence 

 of what he called " bridgeing species." A plant, otherwise 

 immune to the rust experimented with, could be infected by 

 the rust if it had previously been grown on some intermediate 

 or "bridgeing" species. 



E. S. Salmon has demonstrated biological species also among 

 the Erysiphece, the mildew of hops, vines, peas, etc. A species 

 of Erysiphe may grow on a large number of host-plants, and 

 morphologically be always the same, but it cannot easily be 



