PLANTS, WITH GLASS ROOFS. 



217 



511. Ventilation and Aeration. — Till lately, the subject of giving air to 

 plant-houses has been very imperfectly understood ; and, indeed, as it was 

 generally supposed that a very small supply of air was sufficient for the 

 growth of plants, ventilation was principally employed to lower the tem- 

 perature of a hot-house when the heat was too great, or to let off sulphureous 

 or other noxious gases which might be generated by the modes of heating 

 employed. Now, however, that it begins to be well known that plants 

 derive a great proportion of their carbon from the air, another and the 

 most important use of ventilation has been discovered ; and gardeners are 

 become aware that a constant supply of fresh air is almost as necessary to 

 plants as water, and consequently that, without fresh air, no plants can be 

 kept in a perfectly healthy and vigorous state. The admission of air for 

 the purpose of nourishing plants has been very properly distinguished by 

 Dr. Lindley under the name of Aeration, from ordinary ventilation ; and it 

 requires to be regulated in quite a different manner. It has been already 

 observed (253), that if the sashes of a hothouse are opened in front and in 

 the upper part of the roof at the same time, so as to create a thorough 

 draught, when the atmosphere is colder than the temperature of the house, 

 a great injury is done to vegetation, not only by the sudden chill, which 

 the admission of a current of cool air produces, but by the quantity of 

 moisture which it carries off. Hence, aeration should be effected by the 

 circulation of a constant supply of warm moist air (266, 267) ; and hence it 

 is, that plants grown in houses heated by the Polmaise system are generally 

 in a state of vigorous health. Ventilation is, however, frequently necessary as 

 well as aeration. In greenhouses, pits, and frames, where there is a large pro- 

 portion of earthy and moist surface to a small volume of air, the latter may 

 become too moist, and fresh air may be required to dry it; and in every descrip- 

 tion of plant-structure it may be required to lower the temperature. Hence, 

 for houses heated by smoke-flues, and for pits and frames heated by fermenting 

 dung, a greater power of ventilation becomes requisite than for houses heated 

 by hot water in which, noxious vapours can rarely be produced, or the tem- 

 perature raised much above 80° or 90°. For lowering the temperature of a 

 hothouse, air is best admitted by opening sashes or ventilators in the upper 

 part of the roof. In roofs with sliding sashes, the upper sashes along the whole 

 line of roof may be let down uniformly, if the house be at an equal tempe- 

 rature throughout, and rather more at the hottest part, if it is of unequal tem- 

 perature. The width opened need seldom exceed half an inch or an inch in 

 the winter time ; but in summer it may be much greater, according to the 

 temperature to be kept up in the house, and other circumstances. If the roof 

 should be a fixed one, then a narrow opening might be made in the upper 

 angle of the roof along the whole length of the house ; and the cover to this 

 opening might be raised simultaneously and uniformly by lines and pulleys or 

 other means, which need not be here detailed. A portion of the heated air of 

 the house will escape by this opening, while a portion of the outer air will enter 

 to take its place, mixed, as it descends, with the heated air, and becoming, 

 by this means, heated to a certain extent before it reaches the plants. The 

 great object in ventilating houses which are kept at a high temperature is 

 to avoid thorough draughts, which are always produced when ventilators in 

 the front and back are opened at the same time. Even in houses kept at a 

 low temperature, such as greenhouses and conservatories, it is thought 

 desirable in the winter season to admit the air from the roof only, and not 

 from the sides. In summer, when the temperature of the outer air is as 



