224 THE EFFECTS OF DIFFERENT KINDS OF STOCKS IN GRAFTING. 



my experience has enabled me safely to draw, are, that a stock of a 

 species, or genus, different from that of the fruit to be grafted upon it, 

 can rarely be used with advantage, unless where the object of the planter 

 is to restrain and to debilitate: and that where stocks of the same 

 species with the bud, or graft, are used, it will generally be found 

 advantageous to select such as approximate in their habits, and state of 

 change, or improvement, from cultivation, those of the variety of fruit 

 which they are intended to support. 



XXXIII. ON THE VENTILATION OF FORCING-HOUSES. 



[Read before the HORTICULTURAL SOCIETY, May 7th, 1816.] 



IN a memoir which I had two years ago the honour to address to the 

 Horticultural Society *, I stated an opinion that the gardener often 

 erred in the application of heat, by treating his plants as he would wish 

 to be himself treated, and consequently by keeping them much too warm 

 during the night, Experiments, made previously and subsequently to 

 that period, have satisfied me that he as often and as widely errs by too 

 freely admitting the external air during the day, particularly in bright 

 weather. Plants generally grow best, and fruits swell most rapidly, in a 

 warm and moist atmosphere ; and change of air is, to a very limited 

 extent, necessary or beneficial. The mature leaves of plants, and, 

 according to Saussure, the green fruits, (grapes at least), when exposed 

 to the influence of light, take up carbon from the surrounding air, whilst 

 the same substance is given out by every other part of the plant ; so that 

 the purity of air when confined in close vessels has often been found little 

 changed at the end of two or three days by the growth of plants in it. 

 But even if plants required as pure air as hot-blooded animals, the buoy- 

 ancy of the heated air. in every forcing-house, would occasion it to 

 escape, and change as rapidly, and indeed much more rapidly, than would 

 be necessary. 



It may be objected that plants do not thrive, and that the skins of 

 grapes are thick, and other fruits without flavour, in crowded forcing- 

 houses ; but in these it is probably light, rather than a more rapid change 

 of air, that is wanting ; for in a forcing-house, which I have long devoted 

 almost exclusively to experiments, I employ very little fire-heat; and 

 never give air, till my grapes are nearly ripe, in the hottest and brightest 



* See page 213. 



