LIGHT ATTD AIE.] 



PRACTICE OF HOETICULTUR]^. [light and aie. 



wliich will be an eventual destruction of the 

 vital energies. But, on the other hand, if the 

 temperature is kept low, while the amount of 

 atmospheric moisture is considerable, the 

 latter is absorbed without it being possible for 

 the plant to decompose it." 



SOLAR HEAT. 



Perhaps it might be deemed an omission 

 if we did not take notice of the solar rays, 

 being a species of artificial heat, when re» 

 fleeted from other surfaces. Mr, Mackintosh, 

 referring to this subject, says — " This species 

 of heat is materially affected by the admission 

 of the air necessary to the growth and healthy 

 st^te of the plants. Solar heat, if properly 

 regulated by ventilation, is of immense im- 

 portance in the ripening of many of our finer 

 fruits. In the orchard-houses, brought into 

 notice by Mr. Eivers, fruits are ripened by 

 the influence of solar heat alone. The idea is, 

 however, by no means new. We have known 

 many glass-houses in which peaches and grapes 

 have ripened in the greatest perfection, with- 

 out any other heat than that of the sun. 

 . . . . The management of the tempera- 

 ture consists solely in the proper regulation ol 

 the ventilators, securing as much solar heat 

 during the day as possible, ventilating early 

 in the morning, and shutting up early in the 

 afternoon." 



LIGHT AND AIR. 



Whatever may have been the nature of the 

 experiments of natural philosophers in investi- 

 gating the laws of light, and the eflect which 

 the ditFerent coloured rays produce upon the 

 vegetable kingdom, the conclusion arrived at 

 by Mr. Lindley is, that the white light which 

 is natural to plants, is, therefore, the one best 

 adapted to their constitution. Mr. Hunt, 

 who has given much patience to experimenting 

 upon this subject, has still, according to Mr. 

 Lindley, failed in arriving at any valuable 

 practical results in his investigations. " No 

 advantage seems to have resulted from glazing 

 the great palm-house, at Kew, with green 

 glass, of a tint selected by Mr. Hunt himself." 

 In the Tkeonj of Horticulture, it is stated, 

 that "when a leaf ia exposed to the direct 

 influence of the sun, it gives ofi" oxygen by 

 decomposing the carbonic acid; whereupon 

 1)U8 



the carbon remains behind, in the interior of 

 the leaf, in a solid state. In the total absence 

 of solar light, there is little or no extrication 

 of gaseous matter ; and what little is given off 

 will be found to be carbonic acid, which plants 

 exhale at all times in small quantities : oxygen, 

 however, which was before expelled, is inhaled. 

 Hence plants decompose carbonic acid during 

 the day, and acquire it again during the night ; 

 and during the healthy state of a plant, the 

 decomposition of this gaseous matter by day, 

 and its recovery by night, is perpetually going 

 on. The quantity of carbonic acid decom- 

 posed is in proportion to the intensity of light 

 which strikes a leaf, the smallest amount 

 being in shady places ; and the healthfulnes8 

 of a plant is, cceteris paribus, in proportion to 

 the quantity of carbonic acid decomposed ; 

 therefore, the liealtliiness of a plant sliould be 

 in proportion to tJie quantity of light it receives 

 by day." In times when the science of horti- 

 culture was less advanced, no great degree of 

 attention was paid to the form of the roofs of 

 those houses in which plants were artificially- 

 raised. Now, however, the minutest circum- 

 stances enter into the calculations of the 

 scientific gardener. That the greatest pos- 

 sible influx of light might be secured. Sir 

 George Mackenzie proposed hot-houses with 

 curvilinear roofs ; for he thought that " if we 

 could find a form for a glass roof, such that 

 the sun's rays should be perpendicular to some 

 part of it, not on two days, but during the 

 whole year, that form would be the best." 

 Such a form is the sphere ; accordingly, this 

 shape of structure has been, by some, warmly 

 advocated. But, however this may have been, 

 the span and ridge-and-furrow roofs are now 

 the prevailing forms ; and any form is better 

 than that of the lean-to, both for the con- 

 venience of internal arrangement as well as 

 efl'ect. 



As the natural element of plants is the open 

 atmosphere, where they are constantly sur- 

 rounded with air in a state of continual 

 motion, to arrive as nearly as possible at the 

 circumstances of their natural conditions in 

 the climates to which they belong, ought to 

 be the aim of the gardener. " The atmo- 

 sphere is the proper pasture of plants," says 

 Dr. Lindley ; " and its ever- varying density is 

 a natural phenomenon, most intimately con- 



