DISEASED CONDITION OF PLANTS 39 
production and is essential to spore germination, which 
-is the first step towards infection of the host. 
Much general observation and experiment has demon- 
strated the association of fungus and bacterial diseases 
with a moist atmosphere, and many epidemics have 
been intimately connected with abundant rainfall and 
consequent high humidity. In the glasshouse industry 
it is common experience that leaf diseases originate 
during periods of dull, humid weather, when the atmos- 
phere of the houses remains constantly moist. The 
humidity of glasshouse atmospheres is thus important as 
influencing both plant growth and resistance to disease. 
(b) Soil Moisture—Water is essential to life, and 
glasshouse soils, being untouched by the natural rainfall, 
must be artificially watered. 
~The most successful growers are those who have the 
good fortune to possess the faculty of knowing when to 
water and how much to apply at any given time. 
Unfortunately, much of the process is performed without 
an adequate knowledge of cause and effect, and much 
experimental work is necessary to set the practice of 
watering upon a sound scientific basis, for improper 
methods are a fruitful source of harm to glasshouse crops. 
~ An excess of water in the soil fills up all the spaces, 
and there is less room for air in the soil; consequently 
soils saturated with water contain practically no air and 
the plant roots are suffocated. Under such conditions 
the roots cease to function and the plant wilts. Examples 
of this may be found in almost every nursery, where 
plants growing in the sodden patches near to a standpipe 
may be seen in a wilted condition. On digging up these 
plants the roots will frequently be found to be brown 
and decaying. Such dying roots provide a ready means 
of entrance to soil fungi. Should the waterlogged 
conditions persist for any length of time the plant will 
die; but if the soil is allowed to dry out and become 
aerated new roots will form and the plant recovers. 
The dead roots still remain as places of entry for soil 
