2150. Ventilation as applied to plant-houses has long been a subject of great difficulty to gardeners, as the opinions of vegetable physiologists and of practical men seemed to be in direct opposition respecting it. The late Mr. Knight, who was considered the most able vegetable physiologist of his time, had asserted that 'change of air was not either necessary or beneficial to plants, except to a very limited extent,' and all the other writers on the subject followed in his train, till at last the general opinion was that a very small supply of air was sufficient for any plant structure, so far as the growth of the plants was concerned, provided the air of the house were tolerably pure; but where the house was heated by smoke flues, or by fermenting stable dung, it was thought that the air might become charged with sulphureous and other noxious gases, and in such cases a mixture of fresh air might be necessary. It was also thought advisable, in conservatories and frames, where a considerable portion of earth was exposed, to let off the moist vapours arising from it; though the most common purpose for which ventilation was applied, was simply to lower the temperature, when the house felt too hot, by the admission of the atmospheric air.
