314 



FORCING VEGETABLES 



possible to turn the heat into any number of pipes as desired, it is 

 possible to more fully meet the temperature requirements of a 

 given crop in midwinter than in the case of the same crop grown 

 out-of-doors in the summer. During the season when artificial 

 heat is required, the heat is regulated primarily by manipulation 

 of the valves which connect the pipes with the source of heat, 

 cutting off or turning on whatever pipes may be necessary. 

 Incidentally, opening or closing the ventilators will assist in main- 

 taining the proper temperature in case of sudden changes from 

 cloudiness to sunshine or vice versa. However, ventilation of a 

 greenhouse in midwinter is primarily for the purpose of changing 



Fig. 189. — Interior view of large range of lettuce houses near Chicago. 



the air rather than lowering the temperature. In late spring, on 

 warm days, when no artificial heat is required, ventilation may aid 

 materially in keeping the houses from becoming too warm. In 

 still warmer weather, shading of the glass by whitewashing may 

 further aid in keeping the houses cool. Thus the temperature in a 

 glass house can be controlled to a nicet3^ if the house is properly 

 equipped and the operator is skilful. This makes plant growing 

 easy, so far as temperature is concerned. 



Greenhouse Troubles. — However, other difficulties are en- 

 countered. The damp atmosphere of the greenhouse makes 

 conditions especially favorable for the development of fungous 

 diseases on the growing crops; and unless extreme care is exercised 

 in the watering, especially during a siege of cloudy weather, 



