192 DISEASES OF GLASSHOUSE PLANTS 
from the disease in question. Where the disease is 
controlled through the roots they are grown in highly 
infected soil, while if the disease attacks the aerial portions 
of the plants they may be grown in normal soil and 
artificially inoculated by spraying with a suspension of 
spores of the disease. Among these plants, the majority 
of which become diseased, a few remain healthy and are 
selected for seed. From this seed plants are again 
grown under similar conditions and healthy plants again 
selected. Out of the original number only one or two 
healthy plants may be obtained, but after many genera- 
tions of selection the entire batch may be found to be 
healthy and a strain immune to the particular disease 
may be obtained. A necessary detail of the process is 
the exposure of each generation of selected plants to 
copious infection of the disease to resist which the plants 
are being cultivated. 
The selection method has been successfully employed 
in America, where varieties of cotton, cabbage, and 
tomato have been selected which are highly resistant to 
wilt disease due to various species of Fusarium. 
Recently Egerton (19) has elaborated a system of 
selecting in the seed-bed varieties which are resistant to 
attacks of Fusarium lycopersici. Selection has proved 
to be an easy way of obtaining resistant varieties, but 
‘in this country there is need for much investigation in 
this line, especially in relation to glasshouse plants. 
Practically the only resistant variety of any glasshouse 
plants produced in England is Butcher’s Disease Resister 
cucumber, which was selected during an epidemic of the 
leaf spot disease due to Cercospora melonis, and which 
so successfully resisted attacks by the fungus. 
Hybridization 
This process consists in raising a disease resistant 
variety by means of cross-breeding, but is more laborious 
and less certain than that of selection. Usually it is 
