852 REPORT— 1897. 



Thus was discovered the astounding phenomenon of Hetercecism, introducing a 

 new idea into science and clearing up mysteries right and left. 



During the next twenty-five years the number of heteroscious forms has risen to 

 about seventy, including Woronin's recent discovery of this phenomenon in an 

 ascomycete — Sclerotinia heterwcia. 



About 1890 the rust question entered on a new phase. In Australia, India, 

 Sweden, Germany, and America especially, active commissions, inquiries, and 

 experiments were set on foot, and amid some confusion of meaning among some of 

 those concerned much knowledge has resulted from the investigations of Plowright 

 and Soppitt in England; Barclay in India ; Cobb, Anderson, and McAlpine in 

 Australia ; Arthur, BoUey, Smith Ellis, Galloway, Farlow, Harper, and others ia 

 the United States ; Dietel, Klebahn, Sorauer, and others in Germany ; Rostrup la 

 Denmark ; and especially from the continued and indefatigable researches of Eriksson 

 and Henning in Sweden. This renewed work has resulted in the complete con- 

 firmation of De Bary's results, but with the further discovery that our four common 

 cereals are attacked by no less than ten different forms of rust belonging to five 

 separate species or ' form-species,' and with several physiological varieties, and 

 capable of infecting the barberry. Some of these are strictly confined to one or 

 other of the four common cereals, others can infect two or more of them, and yet 

 others can infect various of our common wild grasses as well. 



The fact that what has usually gone by the name of Puccinia graminis is an 

 aggregate of several species is in itself startling enough, but this was not un- 

 expected ; the demonstration that varietal forms exist so specially adapted to their 

 host that, although no morphological diflerences can be detected between them, 

 they cannot be transferred from one cereal to another, points, however, to physio- 

 logical variation of a kind met with among bacteria and yeasts, but hitherto un- 

 suspected in these higher parasitic fungi. It now appears that we must be pre- 

 pared for similar specialisation of varietal forms among TJ&tUaginea as well aa 

 among other Uredince, as follows from the results obtained by Kellermann and 

 Swingle in America, by Klebahn, Tubeuf, and others in Germany, and by Plowright 

 and Soppitt in England. 



Not less remarkable is the conviction that among the many different pedigree 

 varieties of wheat, some are more susceptible to attacks of rust than others. This 

 had often been asserted in general terms, but the extensive observations of Cobb ia 

 Australia, and the even more extensive and exact experiments of Eriksson in. 

 Sweden, seem to put the matter beyond doubt. 



Of course attempts have been made to account for these diflerences in predis- 

 position to the attacks of wheat-rust. 



N. A. Cobb, who has done much for the investigation of Australian wheat- 

 ruets, regards the different susceptibility to rust as due to mechanical causes, and 

 seeks to explain it by the diH'erence in thickness of the cell-walls on the upper and 

 lower leaf-surfaces offering different resistance to the outbreak of the spore-clusters ; 

 the average number of stomata per square millimetre differing in the different sorts 

 of grain, influencing the predisposition to infection ; the presence of waxy bloom 

 affording a protection, and so on. 



Eriksson and Henning have made a critical examination of Cobb's mechanical 

 theory, and show that, for Sweden at any rate, the conclusions of the Australian 

 investigator cannot be confirmed. 



Nevertheless, the problem remains. As matter of fact, different sorts of wheat,, 

 of oats, of barley, and of rye are susceptible to their particular rusts in different 

 degrees, and the question is, Why ? Some complex physiological causes must be 

 at the bottom of it. 



Sorauer pointed out in 1880 that every change of vegetative factors induces 

 differences in composition and form of a plant, and therefore alters the predispo- 

 sition of each individual and variety ; and this applies to the fungus as well as to 

 the host. 



De Bary's proof, in 1886, that a Peziza succeeds in being a parasite only after 

 saprophytic culture to a strong mycelium, that its form is altered thereby, and that 

 probably a poison is excreted, throws side-lights on the same question ; while I 

 myself showed that similar events occur in the case of the lily disease. 



I 



