194 FIXED STRUCTURES FOR GROWING 



iron to conduct away heat in winter, and to attract too much in summer, is 

 also found to be an objection more imaginary than real. It is true that iron, 

 from its being a powerful conductor, is liable to undergo sudden changes of 

 temperature, which must doubtless render it less congenial to plants that come 

 in contact with it than wood or brick ; though plants do not appear to suffer 

 when the iron is in small quantities, such as the rods to which vines are 

 attached under rafters, wire trellis-work, &c. ; but when the rafters are of 

 iron, and when plants are trained round the iron pillars used in supporting 

 hothouse roofs, it may readily be conceived that they will be injured by 

 them. This will also be the case, more or less, when tender plants are grown 

 close under the glass in hotbeds or pits covered with iron sashes. Indeed, 

 when we consider the much greater weight of iron sashes than wooden ones, 

 and the constant occasion that there is for moving the sashes of pits and 

 hotbeds, we would recommend them in most cases to be made of wood. 

 The injury done to plants in the open air by iron coming in contact with 

 them, can only take place when the iron is of considerable thickness ; because 

 we do not find it in the case of cast-iron espalier-rails, or of dahlias, roses, 

 and other open-air plants tied to iron stakes. In plant-houses it probably 

 takes place after the iron has been highly heated by the sun, and then 

 watered, when the chill produced by evaporation will contract the vessels 

 and chill the juices. The greatest objections that we know to iron roofs are 

 the expense and the difficulty of forming them with sliding sashes, which 

 shall not rust in the grooves in which they slide : but this last objection can 

 be obviated, either by forming the styles and rails, or outer frame of the 

 sash, of wood, and the rafters of iron, or the reverse. In the greater pro- 

 portion of plant-houses, however, sliding sashes in the roof may be dispensed 

 with, air being admitted during winter through apertures in the upper angle 

 of the house in the back wall, or by raising a hinged sash in the upper part 

 of the roof; and in the hottest weather in summer, by these and the sliding 

 sashes, or other openings in front. The materials used in the interior of 

 plant- houses, such as shelves for supporting pots of plants, pathways for 

 walking on, walls for enclosing tan or other fermenting matter in pits, are 

 bricks, flagstones, slates, wood, and cast-iron. The paths are sometimes 

 covered with open gratings of cast-iron, which admit of the soil under them 

 being occupied with the roots of vines, climbers, or other plants. Mr. 

 Paxton prefers a flooring formed of loose pieces of board laid across the 

 path ; each piece as long as the path is wide, and about four inches broad, 

 with a one-inch space between. One advantage of this plan is, that the 

 dust and other matters lying on the paths when they are swept, descend 

 immediately without raising a dust in the house to disfigure the leaves of 

 the plants, and encourage the red spider, which dust deposited in the leaves 

 is always found to do. 



488. Heat. The natural heat of the locality is retained in plant-structures 

 by the roof and sides forming a covering which repels radiation from the 

 ground ; and it is increased in them at pleasure, by fermenting substances 

 applied within or externally, by the consumption of fuel, and the convey- 

 ance of the heat so produced in smoke and hot-air flues, by steam, or by hot 

 water in pipes or cisterns. In every mode of supplying heat artificially, the^ 

 following desiderata ought to be kept constantly in view : 1. To maintain a 

 reservoir of heat which shall keep up a sufficient temperature for at least 





