174 SECTIONAL ADDRESSES 
Brefeld and Marshall Ward outlined some of the main features of the 
relations between parasitic fungi and their hosts, but the physiological 
interpretation of fungal parasitism has made greater strides in recent years, 
as was pointed out by Prof. V. H. Blackman in his presidential address to 
this Section in 1924. In particular, the mode of initiation of infection by 
fungus and bacterial parasites has been elucidated in no small degree, and 
the reactions of resistant varieties to attempted invasion by parasitic 
organisms have been interpreted to a considerable extent. 
Before dealing with some of the wider aspects of modern research in 
plant pathology I will mention a few recent investigations which have had 
repercussions in the field of pure botany. Craigie,* in charge of the 
Dominion of Canada Rust Research Laboratory, in carrying out an in- 
tensive study of Puccinia graminis, discoveted that it was heterothallic 
and that its spermatia, hitherto believed to be functionless, were accessory 
‘ fertilising ’ agents. Drayton,® another Canadian plant pathologist, has 
discovered peculiar sexual arrangements in the genus Sclerotinia during 
his investigation of S. Gladioli, the cause of a serious disease of Gladiolus, 
and has demonstrated for this species at least that the microconidia are 
essential for fertilisation, although they were formerly thought to be of no 
importance in the life-cycle. In connection with the grafting of high- 
yielding strains of rubber trees on to seedling stocks in Malaya some failures 
occurred which led Sharples and Gunnery,® pathologists of the Rubber 
Research Institute there, to investigate the processes involved in the union 
of stock and scion, and their results are probably the most complete which 
have been published concerning the manner in which this union is estab- 
lished. A few years ago Tetley 7 compared, from the embryonic condition 
onwards, the development of plum leaves, silvered through the influence 
of Stereum purpureum, with that of healthy leaves, and she has added 
considerably to our knowledge of leaf development as an outcome of these 
investigations. Again, in connection with an intensive study of Silver-leaf 
disease my colleagues and myself ® devised a method of injecting plum 
stems with non-living extracts of Stereum purpureum by means of which 
silvering of the foliage was induced without the intervention of the living 
fungus. This method of injection has obvious applications in certain 
physiological researches and has already been used in a modified form by 
Thomas and Roach ® in investigations on the nutrition of fruit trees. 
These are a few of many illustrations which might be given of the intimate 
relationships between plant pathology and other branches of botany. 
With regard to the wider aspects of recent researches in plant pathology 
I will first deal briefly with progress which has been made in the study of 
the epidemiology of certain parasitic diseases, i.e. the study of the distri- 
bution in space and time of the causative micro-organisms which develop 
epidemically under favourable conditions. The rapid onslaught of a 
parasitic disease on a cultivated crop is a striking phenomenon, and the 
4 Phytopathology, 21, p. 1001 (1931). 5 Mycologia, 26, p. 46 (1934). 
6 Ann. Bot., 47, p. 827 (1933)- 7 Ann. Bot., 46, p. 633 (1932). 
8 Jour. Pomology and Hort. Sci., 5, p. 61 (1926). New Phytologist, 30, p. 128 
(1931). 
9 Jour. Pomology and Hort. Sci., 12, p. 151 (1934). 
