TRANSACTIOXS OF SECTION K. 853 



Reinhardt, in 1892, showed that the apical gTowth of a Peziza is disturbed and 

 interrupted if the culture solution is concentrated by evaporation or diluted ; and 

 Biisgen, in 1893, showed that Botrytis cinerea excretes poison at the tips of the 

 hyphte, 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 germinating 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 hght can 

 be found in the progress of discoveries during recent years. 



You are all no doubt familiar with Pl'etl'er'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 ala:al 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 Engelmann ; he was 

 concerned, not with the proof of oxygen evolution 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 onlj' were these facts confirmed, but it 

 was also shown that this remarkable attraction — chemical attraction, or 'chemotaxis ' 

 — is a very general phenomenon. 



Pfeffer liad already shown that swarmspores of the fungus Saprolcgnia 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 

 hyphse 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 aflbrded by 

 Kihlmann's demonstratit)n of the parasitism of Melanospora on Isaria, where he 

 states that some attractive action exists. In 1882 I had myself seen zoospores of 

 I'ythiian suddenly dart on to the cut surface of a bean-stem, and there fix them- 

 selves. 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 myself, remarked how the phenomena sug- 

 gested that the end of the tube is attracted in some way and by some force which 



