_ DISEASED CONDITION OF PLANTS 35 
decrease in the humidity. Consequently the plants 
transpire more freely and the roots may be unable to 
replace the water lost by the leaves. If this continues 
for any length of time the more tender parts collapse, 
and finally turn brown and wither. The practical 
remedy is to damp the soil surface on bright days follow- 
ing dull weather, and by thus keeping the air moist 
prevent any sudden increase in the rate of transpiration 
of the leaves. 
Tomato fruits suffer from sunburn during prolonged 
periods of intense sunshine. The tissues round the 
stalk end of the fruit become dry, brown, and hard. 
Small cracks appear, giving the surface a rough powdery 
appearance. In most commercial nurseries sunburn 
of the tomato fruit is comparatively rare, and the little 
that does appear may be disregarded, but where it is 
prevalent the houses should be shaded with limewash 
and varieties with dense foliage should be grown. 
Heat 
The relation between temperature and disease must 
be considered with regard to its effect upon the plant 
complex and the pathogen complex, both singly and 
combined. 
The Plant Alone.—The heat requirements of glasshouse 
plants vary considerably, for not only do different crops 
require different temperatures for their maximum 
development, but different temperatures are required 
at different stages of growth. The minimum growth 
temperature is just sufficient to prevent the ces- 
sation of plant processes, which are speeded up as 
the temperature rises, and attain perfection at the 
_ optimum temperature. A further increase in temperature 
upsets the balance of the processes and produces abnormal 
plants. Maximum development, flower and fruit pro- 
duction, and resistance to disease occur at optimum 
temperatures, which do not necessarily coincide; but 
