398 RESTING VEGETATION. 



quence, ill supplied with nutriment, at the period of its ripening, when most 

 nutriment is probably wanted. The muscat of Alexandria, and other late 

 grapes, are, owing to this cause, often seen to wither upon the branch in a 

 very imperfect state of maturity ; and the want of richness and flavour in 

 other forced fruits is, I am very confident, often attributable to the same 

 cause. There are few peach-houses, or indeed forcing- houses, of any kind in 

 this country, in which the temperature does not exceed, during the night, in 

 the months of April and May, very greatly that of the warmest valley in 

 Jamaica in the hottest period of the year: and there are probably as few 

 forcing- houses in which the trees are not more strongly stimulated by the close 

 and damp air of the night, than by the temperature of the dry air of the noon 

 of the following day. The practice which occasions this cannot be right; it ism 

 direct opposition to nature." Physiological and Horticultural Papers, p. 217. 



851. What the night temperature of a hotbed or hothouse ought to be as 

 compared with that of the day, can only be determined by experience ; 

 because plants under glass are so far removed from plants in the free air, 

 that the same difference which takes place in the latter case may not in the 

 former case be advisable. Nevertheless it is clear from the experience of 

 gardeners that a very great fall during the night is seldom or never attended 

 with bad effects, provided there has been sufficient heat and light during the 

 day. Much of the evil of a high temperature during night, especially where 

 opaque coverings are used, must be owing to the absence of light. A scien- 

 tific gardener of great experience observes, " Without extreme caution in the 

 application of coverings to prevent the escape of heat, the worst effects will 

 soon become apparent, I find that, upon the shutters being put on, the internal 

 temperature is raised about five degrees or thereabouts in ordinary circum- 

 stances ;in cases of cold rain or high winds, more ; therefore the injury they 

 cause may probably proceed from this : the plants are inclosed in total dark- 

 ness, with an almost instantaneous and most unnatural increase of tempera- 

 ture, which is in some measure maintained through the night, and the same 

 amount of depression takes place when the coverings are removed and light 

 admitted in the morning. In houses heated by combustion this can in some 

 measure be guarded against, but in those heated by fermenting substances, 

 such as hotbeds, the evil becomes aggravated ; and therefore to structures 

 heated by such materials I cannot see the utility of this application, as 

 economy here cannot be the motive ; materials capable of maintaining a 

 sufficient temperature during a sunless winter's day will in all cases be suffi- 

 cient during night, when a fall of temperature is so beneficial ; yet these 

 structures are covered more than all others, the evils not becoming so appa- 

 rent, possibly because the plants there contained are generally but of 

 annual growth. The debilitating effect of covering houses heated by fire is 

 particularly perceptible in vineries, probably from the position that the 

 plants occupy in the house. Thus, were economy not a material object, 

 and were heating power at command, I certainly should add no covering to 

 the glass roof." G. M. 1812, p. 106. 



852. Double glass roofs would evidently form the least objectionable nightly 

 covering to plant-structures of every kind ; and next to this the use of 

 damaged plate-glass, instead of common crown glass, as from the much 

 greater thickness of the former far less heat would be allowed to escape by 

 conduction. The use of plate glass in cucumber and melon frames, and also 

 in greenhouses and forcinghouses., has of late years been adopted by several 



