HYGIENIC CONDITIONS OF GLASSHOUSES 23 
modern crops are better grown and less susceptible to 
disease than were those in the earliest glasshouses. 
The Heating of Glasshouses 
- While it is common knowledge in a general way that 
_ different crops require different temperatures to enable 
them to resist disease and produce their maximum yield 
there is yet but scanty information as to the exact 
limits of temperature required by each crop. It is, 
however, obviously of great importance that this informa- 
tion should be obtained as soon as possible. Not only 
do different crops require different temperatures, but 
each requires a different temperature at particular stages 
of growth. A plant’s activity reaches its maximum 
at the optimum temperature, other conditions being 
suitable. Below the optimum temperature growth is 
retarded and vitality reduced; while above this 
temperature the plant also suffers, for the rapid growth, 
unaccompanied by proper maturing of the tissues, not 
infrequently leads to death through disease. 
Recent investigations have shown that the temperature 
of the soil is equally as important as that of the air, 
and if the two temperatures are very different the plant 
suffers accordingly. For instance, cucumber plants 
grown in cold, damp beds in a warm house frequently 
wilt badly, and will die if the soil conditions are not 
improved quickly. This is caused by the inability of 
the cold roots to supply the leaves with sufficient water 
to compensate for the loss of water by transpiration. 
The soil and air temperatures have an important bearing 
upon disease. If they are too low or too high the resist- 
ance of the plant is appreciably lowered and it becomes 
an easy prey to disease. On the other hand, the optimum 
temperature for the plant may coincide with that of a 
fungal root or stem parasite which is attacking it, and it 
may be beneficial to lower or raise the temperature slightly 
to make the conditions less favourable for the disease. 
