24 DISEASES OF GLASSHOUSE PLANTS 
Investigations upon temperature requirements are 
thus of great importance, both from the point of view 
of the grower and the pathologist, but unfortunately 
they are expensive to carry out, for specially controlled 
chambers are necessary. 
The present methods of heating glasshouses by means 
of pipes in which either steam or hot water circulates 
are undoubtedly expensive, as is indicated by the fuel 
bill each season, and to those who have studied the 
question it is obvious that many improvements could 
be made. It is a little surprising, perhaps, that the 
various associations of glasshouse growers have not 
enlisted the services of a skilled engineer to study the 
heating question, for this would certainly lead to a 
considerable saving in the cost of heating. 
The Humidity of Glasshouse Atmospheres 
The importance of this factor deserves to be appreciated 
to a much greater extent by glasshouse growers than it 
is at present, for upon it depends such important matters 
as the condition of foliage and flowers, the setting of 
fruit, and the incidence of leaf, stem, flower, and fruit 
diseases. 
As with temperature, so the optimum conditions of 
humidity vary with the kind of plant grown. Ferns 
and plants of similar habitat thrive in an atmosphere of 
high humidity which would result in disease in plants 
like the tomato and vine. If the atmosphere is too dry 
the leaves, flowers, and fruits of most plants are under-. 
sized and hard, but if too moist there is a tendency 
towards a soft, sappy growth possessing a low disease 
resistance. ‘The humidity of the atmosphere is a limiting 
factor in many fungous and bacterial diseases of the 
aerial parts of plants. 
In general, a large proportion of moisture in the air 
is favourable to abundant spore production, while a 
dry atmosphere with suitable air currents is favourable 
F 
