SECTIONAL TRANSACTIONS.—K. 445 
lifetime. Backward extinction is traced at a level 16-6 per cent. for each 
2m.y. and the number of existing species of angiosperms showing senescence 
should be between 2,500 and 250. The BAT curve is based upon observed 
points for Composite, but it applies to Reid’s Pliocene extinctions, Lyell’s 
shell curve, to species-number in angiosperms back to Jurassic and to 
grasses back to Cretaceous. The principles apply also to conifers and 
oligochete worms, but the data for birds indicate modifications of the 
mathematical forms, which may be due to competition between related 
species. 
AFTERNOON. 
Excursion to Rockland and Wheatfen Broads (2.0). 
Friday, September 6. 
_ JOINT Session with Department K* (Forestry). 
Mr, T. R. PEace.—Duich elm disease in Britain (10.0). 
The Dutch Elm disease, which is caused by the fungus Ceratostomella 
ulmi, is most serious in the south-east of England, and decreases in severity 
towards the north and west, and is not known to exist in north England and 
in Scotland. Ina few limited areas it has done severe damage, but over the 
bulk of its range, though common, it has as yet killed few trees. Prophecy 
of the future of the disease is complicated by the very varied rate of progress 
of the attack in different trees, and by the recovery, temporary or permanent, 
of many. In view of these recoveries and of the difficulties of eradication, 
a policy of laissez faire in this country is probably justified. 
This disease, which is probably largely disseminated by bark beetles, 
forms a very good example of the connection between fungi and insects. 
In Scotland, and more rarely in England, other dieback diseases are causing 
considerable damage to mature elms. The symptoms are in most cases 
somewhat different from those of Dutch Elm disease, and the causes are 
as yet obscure. 
Dr. W. J. Dowson.—The watermark disease of the cricket bat willow 
(10.30). 
Approximately 25,000 willows, ten to twenty years old, in Essex, are useless 
for bat making, and are acting as sources of infection of the disease which is 
_ steadilyincreasing. In1922-24 W.R. Day investigated this disease and con- 
_ cluded that the causal agent was a bacterium which he named Bact. Salicis. 
His description of the organism was not complete nor were his successful 
inoculations numerous enough to leave no doubt that his bacterium was the 
true cause. In 1930-32 Miss Lindeijer investigated a similar disease of 
willows, other than S. cerulea, in Holland, and also showed that a bacterium 
was involved but differed in certain characters from Day’s organism. She 
named it Pseudomonas Saliciperda and stated that the two diseases were 
identical. She further concluded that infection was spread by the weevil 
Cryptorrhynchus Lapathi. This insect is rare in Essex and has never been 
found in association with the disease. 
During the last three years the position in this country has been reinvesti- 
gated, the chief result so far being a confirmation of Day’s work. He 
undoubtedly isolated the real bacterial pathogen of the watermark disease, 
although his description needs amendment. 
