CH. IV.] THE LEAVES. 145 



occurs in autumn, and wliich will be more fully con- 

 sidered when the decay of plants is detailed. 



It is the accumulation of carbonic acid and other 

 gaseous matters, such as sulphurous acid and am- 

 monia, which renders ventilation so essential to the 

 health of plants in forcing-pits and hot-houses. 

 They cannot inhale air, overloaded with these con- 

 taminations, without being speedily injured, and the 

 proportions of those gases which rapidly cause dis- 

 ease, or even death, are much less than the gardener 

 usually suspects, for if the sulphurous acid amounts 

 to no more than one cubic foot in ten thousand of 

 the air in a hot-house, it will destroy most of its 

 inhabitants in two days. To avoid such destruction, 

 for the comfort of visitors, and, above all, for the 

 sake of the plant s vigour, air should be admitted as 

 freely as the temperatui'e will permit. The foul 

 warm air can be easily allowed to escape through 

 ventilators in the most elevated parts of the roof, and 

 fresh warm air can be as readily supplied, through 

 pipes made to enter near the flooring of the house after 

 passing through hot water, another source of heat. 



I am quite aware that Mr. Knight has stated that 

 he paid little attention to ventilation, and that plants 

 Avill be vigorous for a time in Wardian cases ; but 

 this does not prove that their Creator made a mis- 

 take when he placed vegetables in the open air. 

 Plants confined in houses or other close structures 



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