CHAPTER X 
WATERING, HEATING, VENTILATING AND 
SHADING 
Success in the management of greenhouse crops de- 
pends more upon the maintenance of proper moisture and 
temperature conditions in both soil and air than upon any 
other factors. Watering, heating, ventilating and some- 
times shading are most vital operations in the growing of 
plants under glass. 
Importance of water.—Most of our greenhouse vege- 
tables contain over 90 per cent of water. The amount of 
water, however, which enters into the composition of 
plants is insignificant compared with that which trans- 
pires from the surfaces of the leaves. The nutrient solu- 
tion in a properly prepared soil is very dilute, and an 
enormous quantity is absorbed by the plant in order to 
meet its food requirements. Water is the vehicle by 
means of which the nutrients are conveyed to different 
parts of the plant, and while some of it enters into the 
composition of the tissues, most of it transpires from the 
leaves. It is likely that the amount of water which daily 
transpires from the leaves of a vegetable plant in a green- 
house, during the bright sunny weather in May or June, 
more than equals the weight of the plant. 
Water also performs various other functions, as render- 
ing plant foods soluble, giving rigidity to plant structures 
and reducing the temperature of plants. Aside from 
these functions relating to plant growth, the humidity of 
the greenhouse, which may be regulated by the grower, 
will depend largely on the use of water. Extremely low 
humidity may be just as harmful under certain condi- 
tions as extremely high humidity. 
149 
