OF VENTILATION. 155 



a low temperature, but will not exist either in dry- 

 cold air or in hot damp air. If the air of cool green- 

 houses is allowed to become damp, the fungi imme- 

 diately spring up on the surface of any decayed 

 leaves, or other matter which may be present, when 

 they spread rapidly to the young and tender parts of 

 living plants ; and when this happens, they consume 

 the juices, choke the respiratory organs, and speedily 

 destroy the object they attack. 



Ventilation is also required in the winter in such 

 places as dung-pits or frames, especially those in 

 which salad, cucumbers, and similar plants are grown. 

 In those cases the object is to dry the air, in order 

 that the plants may not absorb more aqueous par- 

 ticles than they can decompose and assimilate. 

 Although plants of this kind will bear a high degree 

 of atmospherical moisture in summer, when the days 

 are long and the sun bright, and when, consequently, 

 (66, 67,) all their digestive energies are in full acti- 

 vity, yet they are by no means able to endure the 

 same amount in the short dark days of winter, when, 

 from the want of light, their powers of decomposition 

 or digestion are comparatively feeble. Hence, no 

 doubt, the advantage of growing winter cucumbers 

 in forcing-houses, instead of dung-frames. 



One of the causes of success in the Dutch method 

 of winter forcing is, undoubtedly, their avoiding the 

 necessity of winter ventilation, by intercepting the 

 excessive vapour that rises from the soil, and which 

 would otherwise mix with the air. For this purpose 

 they interpose screens of oiled paper between the 



