ee, 
SECTIONAL TRANSACTIONS.—K. 447 
immunity conferred upon a plant by one strain against other strains or 
related viruses and the possibility of using this immunity as a means 
of virus differentiation. Improved methods of ultrafiltration have rendered 
it possible to measure the actual particle-size of viruses. Many problems, 
however, still await solution ; the origin and method of spread of certain 
viruses are unknown and the actual nature of these disease agents is still 
a matter for speculation. 
AFTERNOON. 
Dr. A. S. Horne.— The resistance of the apple fruit to fungal invasion (2.0). 
An account is given of further application of the statistical method of 
studying the course of invasion and its bearing on the nature of disease 
resistance (see Proc. Roy. Soc., B., 102 (1928) ). 
For invasion to take place the fungus concerned must overcome (1) the 
external resistance, and (2) the internal resistance. 
Many fungi are able to penetrate the skin via lenticels or stomata. 
The internal resistance is mainly conditioned by chemical composition 
of the fruit. In general, high acid content tends to check invasion: on the 
other hand, high nitrogen content favours fungal advance. 
Chemical differences between fruit from individual trees, fruit from 
different stocks, or fruit from trees receiving varied manurial treatment are 
reflected in rate of fungal invasion. Statistical treatment of data of radial 
advance has in many instances given results of considerable significance. 
Radial advance is found to be very highly correlated with rate of growth, 
based on microscopical measurement, of fungal hyphz in living sections of 
apple. 
Mr. P. W. Brian.—lInvestigations on the germination of mould spores (2.30). 
In this study the spores of Penicillium expansum Link. and Fusarium 
lateritium var. fructigenum (Fr.) Wr., were used. 
Desiccation of dry spores of these moulds has little deleterious effect ; 
in the case of Fusarium desiccation of previously wetted spores results in 
considerable loss of viability. If the desiccation is only slight the latent 
period of germination is lengthened. 
Fusarium spores, like many others, will germinate in water alone. A 
nutrient causes prolongation of the latent period and increase of the rate of 
subsequent germination. 
For the complete germination of a sample of Penicillium spores suitable 
- sources of nitrogen and carbon, together with a phosphate, are essential. 
Such a simple nutrient will not suffice for continued growth. 
In the case of Penicillium spores there is found to be two distinct stages 
previous to the actual extrusion of the germ tube—a change in the refrac- 
tivity of the spore wall, and swelling of the spore. The occurrence and 
extent of these stages is related to the concentration and composition of the 
nutrient, and to temperature. Under certain conditions germination takes 
place without their occurrence. 
Dr. C. E. Fotster.—The white tip disease of leeks (3.0). 
The White Tip disease of leeks occurs in Scotland and has been found 
near Bristol, causing a serious deterioration of the crop, followed by a rapid 
wilting and rot in the market. ‘The main characteristic is the whitening of 
the leaf tips followed by rotting at or near the leaf bases. 
Q 
