792 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



It is shown that under certain methods of culture, in which the 

 vitality of the host-leaf is interfered with, the restricted powers of infec- 

 tion characteristic of 11 biologic forms " break down. Injury to a leaf, 

 through heat, for instance, rendered it susceptible to the attacks of a 

 "biologic form " which is unable to attack uninjured leaves of the same 

 plant. The following hypothesis is advanced as to the actual manner 

 in which the injury to a leaf causes it to become susceptible to a 

 "biologic form" otherwise unable id infect it. It is supposed that the 

 leaf -cells of each species of host-plant contain a substance or sub- 

 stances — possibly an enzyme — peculiar to each species, which, when the 

 leaf is uninjured and the cells are vigorous, are able to prevent the 

 successful attack of any mildew except the one "biologic form" which 

 has become specialised to overcome the resistance. When the vitality 

 of the leaf, however, becomes affected by injury, this substance is 

 destroyed, or becomes weakened, in the leaf -cells in the neighbourhood 

 of the injury, so that the conidia of other " biologic forms " are now able 

 to infect them. 



The author suggests that injuries to leaves, caused in nature by hail, 

 storms of wind, attacks of animals, &c, may produce the same effect as 

 the artificial injuries described above in rendering the injured leaf sus- 

 ceptible to a fungus otherwise unable to infect it. Conidia produced on 

 these injured places would be able to infect uninjured leaves, and would 

 spread indefinitely. ISuch may be the explanation of a common 

 phenomenon— the sudden appearance of disease caused by parasitic fungi 

 on plants h.therto immune. 



A case is described which, it is believed, gives evidence that the 

 injuries produced by aphides caused leaves previously "immune" to 

 become susceptible. — A. D. C. 



Erysiphacese, On Specialisation cf Parasitism in the. By 



E. S. Salmon (tieih. Bot. Cent. xiv. pp. ii(jl-315). — Details of experiments 

 showing the powers oi infection of Erysiplic graminis on various species. 

 The Oidium on Bromus intcrruptus is capable of infecting fully B. mollis 

 and B. tec tor am, and to a much .ess extent B. brizceformis and B. velutinus, 

 but could not touch twelve other species belonging to its own and other 

 divisions of Bromus. An Oidium on B. hordcaceus caused full infection 

 on B. mollis and two other species, " subinfection " on three species, and 

 failed on thirteen other species. The Oidium of Trifolinm pratense was 

 also found to be a true " biologic form " specially adapted to this species. 

 Other experiments are detailed. Larvae were discovered feeding on 

 Sphcerothcca, mildew of Gooseberries, and also at Cambridge and other 

 places, but most unfortunately it could only be discovered that they 

 belonged to a ily, Atjromyza Bp., or to some closely allied genus. The 

 author raises the interesting question as to whether residence of the 

 fungus on some special host-plant affects its power of infection on other 

 species. 



Neger considers that the ascospores of two " biologic forms " of Erysiphe 

 are probably able to infect both host-plants, whilst the conidia (Oidium) 

 adapts itself very quickly to any particular substratum. Salmon's experi- 

 ments were carried out in the laboratory. — G. F. S.-E. 



