OV BESTING. 329 



operates upon the excitability of vegetation ' at that 

 period of twenty -four hours, when, from othe'r causes, 

 the powers of digestion and assimilation are suspend- 

 ed. As far as is at present known, that power is 

 heat ; and therefore we must suppose that, to main- 

 tain at night in our hot-houses a temperature at all 

 equal to that Of the day, is a practice to be much con- 

 demned. Plants will no doubt lengthen very fast at 

 night in a damp heat, but what is at this time produced 

 seems to be a mere extension of the tissue formed 

 during the day, and not the addition of any new part ; 

 the spaces between the leaves are increased, and the 

 plant becomes what is technically and very correctly 

 called drawn; for, as has been justly observed, "'the 

 same quantity only of material is extended to a greater 

 length, as in the elongation of a wire." 



Mr. Knight has pointed out another ill effect of 

 high temperature during the night, namely, that it 

 exhausts the excitability of a tree much more rapidly 

 than it promotes its growth, or accelerates the matu- 

 rity of its fruit ; which is, in consequence, 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 as he 

 thinks 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," he adds, "or indeed forcing-houses 

 of any kind, in this country, in which the tempera- 

 ture does not exceed, during the night, in the months 



