36 DISEASES OF GLASSHOUSE PLANTS 
any deviation from the particular optimum produces 
an adverse effect proportional to the extent of deviation. 
The temperature of the soil is just as important as 
that of the atmosphere, and both must be taken into 
account when considering temperature effects. Harmful 
effects are produced in the plant when these two tempera- 
tures do not approximate to one another, as instanced 
by the wilting of cucumbers planted in cold beds in a 
warm house. The chilled roots are unable to absorb 
water from the soil with sufficient rapidity to replace 
that evapora ted from the leaves. Similar adverse effects 
upon the physiological processes, while not so obvious 
as this, are produced whenever an appreciable difference 
occurs between the two temperatures, the effects being 
proportional to the difference between them. In con- 
sequence the plant’s vitality is impaired and its power 
to resist disease is reduced. Generally speaking, 
temperatures between the optimum and maximum tend 
to the development of soft, sappy growth and an increased 
susceptibility to disease, while those below the optimum 
encourage slow, hard growth accompanied by an in- 
creased resistance to disease. Exceptions, however, are 
found in respect of certain vascular diseases, to which 
hard, slow growing plants with thin stems are more 
susceptible than soft, rapidly growing plants with 
thicker stems. 
The Pathogen Alone.—The fungus or bacterial parasite 
reacts to temperature in much the same way as the plant, 
and possesses minimum, optimum, and maximum tem-— 
peratures for growth. As a general rule fungi can 
survive very low temperatures. Most fungi are uninjured 
by exposure to a temperature of 0° C. and can be frozen 
solid without undue harmful results. Fungus spores are 
not destroyed to any useful extent by the normal winter 
conditions which prevail in this country. 
Similarly, many fungi and bacteria are able to survive 
exposure to high temperatures. The thermal death 
point of some fungi has been found to be as high 
