HYGIENIC CONDITIONS OF GLASSHOUSES — 25 
to the efficient dispersal of the spores. The presence 
of a film of moisture on the surface of the plant enables 
the spores to germinate and enter the tissues. Con- 
sequently high humidity should be avoided when plants 
are exposed to infection. 
An intimate relationship exists between the humidity 
_ and the temperature, for an increase in the latter produces 
a proportional decrease in the former. The humidity 
in glasshouses of the taller types is generally less than 
that in low houses, for the larger volume of air in the 
former requires more water to raise it to a given degree 
of humidity than the smaller volume in the latter. 
Consequently leaf diseases are the more easily controlled 
in houses with high gutters and ridges. 
Watering 
The process of watering plants is probably one of 
the most important of all horticultural practices. Many 
experienced growers have reduced it to a fine art, but 
their methods are largely intuitive and difficult to explain. 
Insufficient watering prevents the plant from reaching 
its maximum development and produces physiological 
disorders, but error on this side is preferable to watering 
in excess. Excessive watering leads to waterlogged soil 
conditions, except on extreme sandy soils. An excess of 
water in the soil produces a deficiency of air about the 
plant roots, and serious complications result. Such 
conditions produce a reduction in yield and resistance 
to disease, and in extreme cases death by suffocation. 
The careless splashing of water about the base of plants 
causes harm in many ways. It may chill the plants and 
so reduce their vitality ; it changes the physical nature 
of the soil, which puddles and dries hard at the surface ; 
and it leaves the lower foliage hanging in water, pro- 
viding ideal conditions for infection by certain organisms. 
It increases the rate of spread of such diseases as “ buck- 
eye rot” of the bottom trusses, “foot rot” and 
