Moisture and Water Requirements 75 



From Table 12 it is evident that heat lowers the 

 humidity. It is also to be noted that the decrease in 

 humidity is rapid on bright days even when most 

 cold. It is to be further noted that not only the 

 heat, but the ventilation of the house during the 

 day when the heat is turned on in the pipes, tends 

 to reduce the humidity still further. Rose growers, 

 for instance, are aware that at the approach of 

 spring the plants suddenly improve greatly. This 

 is generally attributed to longer and brighter days. 

 However, according to Blake, this change is due to 

 an increase in humidity in the hothouse due to the 

 shutting off of the heat in the pipes. 



Effect of Humidity on Greenhouse Crops 



For further information on this subject, we have 

 to refer again to the researches of Blake. He finds 

 that the American Beauty rose, for instance, as a 

 result of insufficient humidity fails to produce new 

 shoots from the base of the plant. Moreover, the 

 foliage, except at the tips of the growing shoots, be- 

 comes hardened and toughened. The older leaves 

 turn yellowish and fall off prematurely, leaving 

 bare stalks. As a result of this the younger and 

 smaller rootlets die out. Blake also found that a 

 high humidity tends to increase the size of the foli- 

 age, and the flowers seem to be likewise favorably 

 affected. With low humidity the leaves of small 

 rose plants wilt on bright days, even though the 

 soil is kept moist. Frequently the leaves turn black, 



