56 DISEASES OF GLASSHOUSE PLANTS 
eight tiny special spores or ascospores. The pycnidia 
and perithecia do not open except when conditions are 
favourable, and then there are often special devices for 
ejecting the spores from the inside (Fig. 3). 
t. Root Diseases 
“DAMPING OFF”? OF TOMATO SEEDLINGS 
Symptoms.—Typically the plants are attacked at the 
soil level (Fig. 4), where the tissues are browned, become 
Fig. 4. Tomato seedling attacked by ‘‘ damping off ”’ disease at a. 
softened, and collapse, causing the plant to fall over. 
The disease spreads rapidly from seedling to seedling in 
the film of water covering the soil surface. ‘Thick 
sowing is to be deprecated, for this assists the spread of 
the disease, as shown by the following experiment. 
Seeds were sown in infected soil in thicknesses varying 
from 600 to 25 seeds per box, in sets of four boxes at 
each degree of thickness. In half the boxes the seedlings 
