PLANTS, WITH GLASS ROOFS. 217 



611. Ventilation. Plants do not require large supplies of fresh air for the 

 purpose of respiration like animals, because while the latter speedily render 

 air impure, by depriving it of oxygen, and giving off carbonic acid, by the 

 former oxygen is given off, and the carbonic acid of the atmosphere inhaled. 

 A very small supply of air, therefore, is sufficient for any plant-structure, as 

 far as the growth of plants is concerned, provided the air of the house be 

 tolerably pure ; but where the air is heated by smoke-flues or by fermenting 

 stable dung, it may be charged with sulphureous or other noxious gases ; 

 and, in such cases, a frequent mixture of fresh air may be necessary. 

 In greenhouses, and pits, and frames, where there is a large proportion 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 ; while in every description 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, under ordinary circumstances, noxious vapours can- 

 not be produced, or the temperature raised much above 80 or 90. In many 

 cases the small quantity of fresh air required for such houses will find its 

 way into the house through crevices in the roof or sides, and by the occasional 

 opening of doors by the going hi and out of the gardener. Where larger sup- 

 plies of fresh air are requisite, a portion of the roof or sides, or of both, must 

 be made to open, and the extent to which this is effected will depend on the 

 dimensions of the house, its uses, and other circumstances. The common 

 purpose for which ventilation is employed, is to lower the temperature of the 

 house, and this is generally done by opening sashes in front, and in the upper 

 part of the roof, thereby creating a draught of cool air through the house, 

 most injurious to vegetation, by the sudden chill which it produces, as well 

 as its dryness (252, 260, 267). Only a small quantity of outer air is at most 

 seasons requisite for lowering the temperature of a hothouse, and this can be 

 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 temperature throughout, 

 and rather more at the hottest part, if it is of unequal temperature. The 

 width opened need seldom exceed half an inch or an inch in the winter time ; 

 but in summer it may be much larger, 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 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, mixing 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 tis greenhouses and conservatories, it is always desirable in the winter 

 season to admit the air from the roof only, and not from the sides. In sum- 

 mer, when the temperature of the outer air is as high as that of the house 

 ought to be, openings may be made in every direction at pleasure. In 

 stoves the precaution of covering the openings of the upper part of the roof, 



