LIGHT CONSIDERED, WITH REFERENCE TO HORTICULTURE. 89 



heat, in properly managed houses, acquire a flavour superior to those grown 

 in plant houses, where the air is kept moist for the sake of the plants." 

 In corroboration of this, the same gardener mentions an instance in 

 which " in forcing an old house of vines a continual current of air was 

 admitted at the end where the fire entered, in order to maintain the tem- 

 perature at both ends of the house nearly alike. At the end of the 

 house where so much air was admitted, invariably, till the present year, the 

 most abundant, finest, and best coloured grapes, have been produced ; but in 

 the present year the case has been materially different, in consequence of 

 one of Dr. Arnot's stoves being placed at the other end of the house, by 

 which the necessity of admitting air at the usual place, and to the usual 

 extent, became unnecessary. The difference in the colour and quality of 

 the grapes between the two ends of the house is now inappreciable." (Gard. 

 Mag., vol. i., third series, p. 25.) It will be observed, that in this case the 

 air was heated before entering the house, which the writer represents as 

 essentially necessary. " Good grape-growers," Mr. Duncan adds, " seldom 

 admit a current of air directly from the atmosphere, except in extremely 

 warm weather, and, even then, never through a doorway, unless it be situ- 

 ated at the back of the house, where the temperature is in general higher 

 than in front : to admit air in front, unless in very mild weather, would be 

 most injurious to the plants." 



275. It is certain that in all countries the climate, during the growing sea- 

 son, is moist, and at the ripening season comparatively dry, and hence the 

 practice of withholding water from fruit-bearing plants under glass, when 

 the fruit is ripening, is in direct imitation of nature. It is also natural to 

 suppose, that in the ripening season in the open air, when the surface of the 

 soil is dry, the atmosphere over it will be less saturated with vapour than 

 when the soil is moist ; and hence, the recommendation of dry air for the 

 maturation of fruits is also natural. The effect of this air must be greatly 

 to increase the perspiration of the leaves, which is probably favourable to 

 the increased action of solar light, in the production of the saccharine mat- 

 ter, and the peculiar odoriferous properties of fruits. Where growth, and 

 not the maturation of fruit, is the object, more water in the leaves appears 

 necessary, probably to aid in the production of carbon. 



276. It will be obvious, from the foregoing remarks, that the mode of 

 admitting air to hothouses, by a range of ventilators in front, and a corres- 

 pondent range at the back, must be highly injurious to the plants in the winter 

 season; and, indeed, more or less so at all seasons, when there is much 

 difference between the temperature of the open air and that of the house 

 to be ventilated. 



277. Indeed, cultivators may lay it down as a general principle, that 

 neither water nor air ought to be given to plants at a much lower tempera- 

 ture than that of the soil in which they grow, or the air by which they are 

 surrounded. 



SECT. IV. Light) considered with reference to Horticulture. 



278. Light, as we have seen (143), is one of the most important agents 

 in the growth of plants. It is to light they owe their green colour, and 

 the maturation of their fruits. When plants are grown in situations where 

 they obtain no light, as in dark cellars, instead of that beautiful variety of 

 colours, and of properties, which they present when grown exposed to the 



