Host and Parasite in certain Diseases of Plants. 



443 



main intact for weeks or months. No doubt in these cases also the 

 •entrance of the hyphae or haustoria into the tissues is aided by any 

 factors which cause the cell-walls to be softer or thinner than in the 

 normal condition ; and it is certain that many failures by those who 

 have experimented with Uredinous fungi are attributable to their 

 sowing the spores on older, well matured tissues. 



We are here, however, abandoning the subject of the present 

 lecture, because, in the first place, the phenomena just referred to 

 appertain to sporadic rather than epidemic diseases, and because, 

 secondly, they tend to the subject of symbiosis proper, where the rela- 

 tions between the host and the parasite have become so arranged that 

 both may be said to benefit by the commensalism, as exemplified in 

 the lichens, and some of the recently described cases of mutualism 

 between fungi and the roots of Phanerogams. 



April 17, 1890. 



Sir G. GABRIEL STOKES, Bart., President, in the Chair. 



The Presents received were laid on the table, and thanks ordered 

 for them. 



The following Papers were read : — 



I. ei Preliminary Note on Supplementary Magnetic Surveys of 

 Special Districts in the British Isles." By A. W. ROoker, 

 M.A., F.R.S., and T. E. Thorpe, Ph.D., B.Sc. (Vict.% F.R.S. 

 Received March 5, 1890. 



During- the summer of 1889 we carried out additional magnetic 

 surveys of the Western Isles and the West Coast of Scotland, and of a 

 tract of country in Yorkshire and Lincolnshire. 



Both districts were selected with special objects in view. We had 

 found that powerful horizontal disturbing forces acted westwards 

 from the Sound of Islay, from Iona, and from Tiree, and we bad 

 deduced a similar direction for the disturbing force at Glenmorven 

 from Mr. Welsh's survey of Scotland in 1857-58. The whole district 

 presents peculiar difficulties, partly from the fact that local disturb- 

 ance is likely to mask the effects of the regional forces, partly because 

 the normal values of the elements must be especially uncertain at 

 stations on the edge of the area of our survey. 



If, then, the general westward tendency of the horizontal disturb- 



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