RESTING VEGETATION. 365 



found growing where the temperature varies little, owing to shade and 

 shelter, the vicinity of springs, &c., but these are only the exceptions. 



Nightly temperature requires to be considered chiefly with reference 

 to plants under glass. The fear of too low a temperature within being 

 produced by the cold without, has naturally led gardeners to bestow 

 particular care on covering up hotbeds, and raising the temperature of 

 the air in hothouses in the evenings. In consequence of this, it often 

 happens that when the temperature of the external air has not fallen so 

 low during the night as was expected, the temperature under glass 

 becomes greater than was intended. The consequences of this excess 

 of heat during the night are highly injurious to the fruit trees of 

 temperate climates, and not at all beneficial .to those of tropical climates; 

 for the temperature of these is, in many instances, low during the night. 

 In Jamaica, and other mountainous islands of the West Indies, the air 

 upon the mountains becomes soon after sunset chilled and condensed, 

 and, in consequence of its superior gravity, descends and displaces the 

 warm air of the valleys ; yet the sugar-canes are so far from being 

 injured by this sudden decrease of temperature, that the sugars of 

 Jamaica take a higher price in the market than those of the less 

 elevated islands, of which the temperature of the day and night is 

 subject to much less variation. Plants, it is true, thrive well, and 

 many species of fruit acquire their greatest state of perfection in some 

 situations within the tropics where the temperature in the shade does 

 not vary in the day and night more than- seven or eight degrees ; but 

 in these climates, the plant is exposed during the day to a full blaze of 

 a tropical sun, and early in the night it is regularly drenched with 

 heavy wetting dews; and consequently it is very differently circum- 

 stanced in the day and in the night, though the temperature of the air 

 in the shade at both periods may be very nearly the same. A high 

 night temperature often causes a large portion of the blossoms of 

 cherries and other fruit trees to prove abortive, because they are 

 forced to expand before the sap of the tree is properly prepared to 

 nourish them. Under a lower night temperature, the blossoms advance 

 with more vigour, and expand to a larger size. Another evil effect of 

 a high temperature at night is that it exhausts the tree rather than 

 hastens the growth or accelerates the maturity of the fruit. The 

 Muscat of Alexandria, and other late grapes, are, probably 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 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 is in direct opposition to nature. 



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



