CHEMOTROPISM 275 



impressed with it, but nothing short of actual demonstration 

 ever convinced him ; and when he proceeded to investigate the 

 actual histological facts on which the theory rested he promptly 

 exploded it. 



It is interesting to note that Ward, as I know from corre- 

 spondence at the time, had himself been embarrassed in investi- 

 gating the Ceylon coffee disease by the same kind of appearance 

 which had misled Eriksson. It is due to an optical fallacy. 

 When the hypha of a uredine attacks a cell it is unable to 

 perforate it with its whole diameter. It infects it, however, with 

 a reduced and slender filament ; this expands again after per- 

 foration into a rounded body, the haustorium. In a tangential 

 section the perforating filament cannot be distinguished, and the 

 haustorium looks like an independent body immersed in the 

 cell-protoplasm and with no external connection. It requires a 

 fortunate normal section to reveal what has really taken place. 

 Ward was accordingly able, in a paper in the Phil. Trans, in 

 1903, to dispose conclusively of the mycoplasm. This cleared 

 the ground of an untenable hypothesis. The complicated nature 

 of the problem which still presented itself for investigation can 

 only be briefly indicated. Sir Joseph Banks, whose scientific 

 instinct was sound but curiously inarticulate, had pointed out 

 that the spores entered the stomata, and warned farmers against 

 using rusted litter. Henslow, one of Ward's predecessors in the 

 Cambridge chair, had been confirmed by Tulasne in showing 

 that the uredo- and puccinia-spores (of the barberry) belonged 

 to the same fungus. De Bary traced the germination of the 

 spores and the mode in which the hyphae invaded the host ; 

 the fundamental fact, which he observed but did not explain, 

 was that the germinal filament, after growing for a time super- 

 ficially, bent down to enter the tissues of its host. Pfeffer in 

 1883 discovered chemotaxis, the directive action of chemical 

 substances on the movement of mobile organisms. De Bary 

 had previously hinted that the hypha might be attracted by 

 some chemical ingredient of the host plant. Myoshi, a pupil of 

 Pfeffer's, showed finally in 1894 that if a plant were injected by 

 a chemotropic substance a fungus -hypha not ordinarily parasitic 



might be made to behave as such and attack it. 



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