S. p. Wiltshire 347 



touch the juncture of the epidermal cells would be bounded by a line, 

 the breadth of the appressorium within the edge of the cell ; and taking 

 the relative sizes of the diameters of the cells and appressoria roughly 

 as 6 : 1, we should find this area would be f that remaining. Hence 

 we might expect the appressoria to touch the junctures of the epidermal 

 cells more often than not. On the other hand, however, other fungi 

 as well as bacteria are said to show a similar liking for that part of the' 

 cuticle above the junctions of the walls. The elaborate paper of Fulton, 

 in which he calls in question the whole of the work done on chemotropic 

 attraction of fungal hyphae, makes us suspicious of accepting altogether 

 the works of earlier investigators and it becomes more and more evident 

 that a further study of chemotropism is necessary for the progress of 

 enquiry into the question of the attack of parasitic fungi. It is not yet 

 estabUshed whether chemotropism does exist at all; and not until we 

 know, not only whether it exists but also whether it plays any part in 

 infection, can we hope to attempt to solve the question why the fungal 

 hypha enters the host. 



If some chemotropic substance does influence the direction of growth 

 of the germ tube, it must either be the exterior layers of the cuticle or 

 something formed interior to them and then excreted. There is no 

 reason to suppose that the exterior of that portion of the cuticle covering 

 the perpendicular epidermal walls has peculiar properties. Nordhausen 

 has suggested that the mechanical checking which the germ tube 

 undergoes on crossing a groove or the collection of water in it may 

 account for the predilection of the appressoria for this position, yet the 

 behaviour of appressoria is quite similar on varieties like Port and 

 William's Bon Chretien which have no grooves (see Figs. 26, 27 and 

 compare Fig. 28). That the cuticle which covers the junctures of the 

 epidermal cells is, however, different from the remaining is shown by 

 the different staining properties it possesses; this fact, taken in con- 

 junction with the occasional occurrence of hyphae embedded there, 

 suggests rather pointedly that this may be a substance chemotropically 

 attractive to the germ tube. At the best, however, this chemotropism 

 can be only an accessory factor in infection since hyphae can penetrate 

 the cuticle quite well in the middle of the epidermal cell-walls. 



The growth of the infection hyphae from the appressoria through 

 the cuticle itself is interesting in connection with the part played by 

 chemotropism. If the hypha entered solely because attracted by the 

 substances in the epidermal cells or cell-walls of the host, we should 

 suspect that it would bore directly through the cuticle to reach the 



