22 DISEASES OF GLASSHOUSE PLANTS 
of the modern glasshouse are an immense improvement 
upon those of the earliest houses, so advancement has 
been made in air capacity and ventilation. 
There is still, however, much to be done in the latter 
respect, for the ventilation of modern glasshouses is far 
from satisfactory from a plant disease standpoint. It 
is true the air capacity has been greatly increased by the 
building of higher and larger houses, but the methods 
of circulating the air need much improvement. It is 
a well-known fact that the incidence of many diseases is 
so intimately connected with stagnant air rich in water 
vapour that they may be greatly reduced, and in many 
cases entirely checked, by increasing the amount of 
ventilation. 
Necessary investigations should be concerned with a 
study of the number, size, and position of ventilators in 
houses of different shapes and sizes which are required 
to produce the correct number of complete changes of 
air in any given time. There is evidence to show that 
the mere movement of air in a house, apart from intro- 
ducing fresh air and expelling the old air, has a beneficial 
effect upon the plant, and adequate experiments should 
be arranged. The importance of the relation between 
the cubical contents of a house and the area covered 
is apparent from several standpoints. High houses, 
exemplified by the “aeroplane” type of tomato house 
(14 or 15 feet wide, 7 feet to the gutter, and 11 feet to 
the ridge), allow unrestricted development of the plant, 
the tops of which in low houses become congested 
towards the end of the season. Another effect of abun- 
dant head space is improved ventilation and circulation 
of the air. 
On the other hand, the larger volume of air in houses 
of the high type requires more heat to raise it to a given 
temperature and maintain it at this than does the smaller 
volume of a low house. The evolution of the larger, 
better lighted glasshouses has produced a marked change 
in methods of crop management, with the result that 
