September i6, 1897] 



NATURE 



481 



spores, and showed that P. rtibigo-vera alternates on Boragineae 

 as jEc. asperifolit, while P. coronata, separated by Corda in 

 1837, does the same as .Ec. Rhainni on Rhamnus. 



Thus was discovered the astounding phenomenon of Heter- 

 (Kcism, introducing a new idea into science and clearing up 

 mysteries right and left. 



During the next twenty-five years the number of hetercecious 

 forms has risen to about seventy, including Woronin's recent 

 discovery of this phenomenon in an ascomycete — Sckrotinia 

 hetercfcia. 



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 con- 

 cerned much knowledge has resulted from the investigations of 

 Plowright and Soppitt in England ; Barclay in India ; Cobb, 

 Anderson, and McAIpine in Australia ; Arthur, Boliey, Smith 

 Ellis, Galloway, Farlow, Harper, and others in the United 

 States ; Dietel, Klebahn, Sorauer, and others in Germany ; 

 Rostrup in Denmark ; and especially from the continued and 

 indefatigable researches of Eriksson and Henning in Sweden. 

 This renewed work has resulted in the complete confirmation 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 unexpected ; the demonstration that 

 varietal forms exist so specially adapted to their host that, 

 although no morphological differences can be detected between 

 them, they cannot be transferred from one cereal to another, 

 points, however, to physiological variation of a kind met with 

 among bacteria and yeasts, but hitherto unsuspected in these 

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

 pared for similar specialisation of varietal forms among 

 UsnlagineiE as well as among other Uredincc, 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 in 

 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 

 differences in predisposition to the attacks of wheat-rust. 



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

 Australian wheat-rusts, regards the different susceptibility to 

 rust as due to mechanical causes, and seeks to explain it by 

 the difference 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 predisposition 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. 



NO. 1455, VOL. 56] 



Reinhardt, in 1892, showed that the apical growth of a 

 Peziza is disturbed and interrupted if the culture solution is 

 concentrated by evaporation or diluted ; and Bllsgen, in 1893, 

 showed that Botrytis cinerea excretes poison at the tips of the 

 hyphre, confirming my results with the lily disease in 1888, and 

 that a similar excretion occurs in rust-fungi. 



De Bary had also shown, in 1886, that the water-contents of 

 the infected plant influence the matter ; and I may remark that 

 we have here also to consider the case of Botrytis attacking 

 chrysanthemums, &c., in autumn, with respect to the chilling 

 of the plant, which lowers the vitality of the cells and causes 

 plasmolysis, as well as the fact that cold increases the germin- 

 ating capacity of spores, as Eriksson showed. 



I discussed these points at some length a few years ago in the 

 Croonian Lecture to the Royal Society, and it now remains to 

 see if any further gleams of light can be found in the progress 

 of discoveries during recent years. 



You are all no doubt familiar with Pfeffer's beautiful work on 

 chemotaxis, and with the even more fascinating experiments of 

 Engelmann, which prove that bacteria will congregate in the 

 neighbourhood of an algal cell evolving oxygen. 



When Pfeffer took the matter up in 1883, he was interested 

 in the question as to the stimulating action of various bodies on 

 mobile organisms, for he found that many motile antherozoids, 

 zoospores, bacteria, &c., when free to move in a liquid, are 

 vigorously attracted towards a point whence a given chemical 

 substance is diffusing. 



Pfeffer's problems had nothing to do with those of Engel- 

 inann ; he was concerned, not with the proof of oxygen evolu- 

 tion or the movements of bacteria as evidence of the presence 

 of that element, but with a fundamental question of stimulation 

 to movement in general. 



Pfeffer found that the attractive power of different chemical 

 substances varies according to the organism, and according to 

 the substance and its concentration. He also showed that 

 various other bodies besides oxygen thus attract bacteria — e.g. 

 peptone, dextrose, potassium salts, &c. These experiments are 

 by no means difficult to repeat, and are now employed in our 

 laboratories. 



During the course of several years not only were these facts 

 confirmed, but it was also shown that this remarkable attraction 

 — chemical attraction, or "chemotaxis'''' — is a very general 

 phenomenon. 



Pfeffer had already shown that swarmspores of the fungus 

 Saprolegnia are powerfully attracted towards the muscles of a 

 fly's leg placed in the water in which they are swimming about, 

 and pointed out that in many cases where the hyphre of fungi 

 suddenly and sharply bend out of their original course to enter 

 the body of a plant or animal, the cause of the bending lies in 

 a powerful '' chemotropic" action due to the attraction of some 

 substance escaping from the body. 



This idea of an attractive action between the living substance 

 of two organisms growing in close proximity was not entirely 

 new — it was, so to speak, in the air — e.g. the fusions of mycelial 

 cross-connections and clamp-organs, and of the spores of 

 Tilletia, Entyloma, &c. One of the most striking examples is 

 afforded by Kihlmann's demonstration of the parasitism of 

 Melanospora on Isaria, where he states that some attractive 

 action exists. In 1882 I had myself seen zoospores ol Pythium 

 suddenly dart on to the cut surface of a bean-stem, and there 

 fix themselves. But it is due to Pfeffer and his pupil Miyoshi 

 to state that they were the first to demonstrate these matters 

 clearly. 



To understand the important consequences which followed, 

 I must now refer to another series of discoveries. 



When a spore of a parasitic fungus settles on a plant, it 

 frequently behaves as follows. The spore germinates and forms 

 a slender tube of delicate consistence, blunt at the end and 

 containing colourless protoplasm. De Bary long ago .showed 

 that such a tube — the germinal hypha — only grows for a short 

 time along the surface of the organ, and its tip soon bends down 

 and enters the plant, either through one of the stomata or 

 by boring its way directly through the cell-walls. Several 

 observers, and among others my.self, remarked how the 

 phenomena suggested that the end of the tube is attracted in 

 some way and by some force which brings its tip out of the 

 previous direction, and De Bary even threw out the hint that 

 this attraction might be due to some chemical substance excreted 

 by the host plant. I myself showed that the condition of the 

 attacked plant affected the ease with which the tube penetrates 



