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. 



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