HYGIENIC CONDITIONS OF GLASSHOUSES 29 
of such remains, which is to burn them immediately 
they are removed from the houses. Every nursery 
should possess an incinerator for this purpose, and the 
beneficial results of its use will far more than compensate 
for the initial cost. 
Sources of Infection of Plant Diseases 
(a) The Water Supply.—The possibility that con- 
taminated water supplies are a frequent source of 
infection to glasshouse crops was indicated in 1919, 
when there were attacks of “damping off” of tomato 
seedlings, caused by Phytophthora cryptogea, and “ buck- 
eye rot” of tomato fruits, caused by Phytophthora 
parasitica. 
By spraying healthy seedlings and bunches of fruit, 
under properly controlled conditions, with samples of 
the suspected waters it was possible to reproduce the 
diseases in question, and it thus became evident that a 
centre of infection existed in certain nursery waters. 
A suitable method of examining water supplies was 
devised by filtering large volumes through special wire 
and cotton wool filters, and a systematic examination 
of the waters used in nurseries in the Lea Valley district 
was made. 
Important results (8), were obtained, for it was demon- 
strated that while water from some sources was free 
from contamination, that from others contained many 
fungal and bacterial parasites of glasshouse crops. 
Water from the Water Company (Metropolitan Water 
Board) and deep artesian wells was generally free from 
contamination. Samples from wells receiving surface 
drainage contained large numbers of plant pathogens, 
deep wells of this kind being only slightly less con- 
taminated than shallow wells; but shallow wells placed 
some distance away from the glasshouses were generally 
purer than those surrounded by houses. Wells whose 
waters contained a large amount of decaying organic 
