SECT. III. CAUSES OF THE SAP's ASCENT. 



turns ; because the sap is sometimes found to flow 

 on the south side of a tree before it flows on the 

 north side, that is, on the side exposed to the in- 

 fluence of the sun's heat sooner than on the side 

 deprived of it ; because plants may be made to 

 vegetate even in the winter, by means of forcing 

 them in a hot-house ; and because plants raised 

 in a hot-house produce their fruit earlier than such 

 as vegetate in the open air. 



There can be no doubt of the great utility of 

 heat in forwarding the progress of vegetation ; but 

 it will not therefore follow that the motion and 

 ascent of the sap are to be attributed to its agency. 

 On the contrary, it is very well known that if the 

 temperature exceeds a certain degree, it becomes 

 then prejudicial both to the ascent of the sap and 

 also to the growth of the plant. Hales found that 

 the sap flows less rapidly at mid-day than in the 

 morning ; * and every body knows that vegetation 

 is less luxuriant at mid-summer than in the spring. 

 So also in the case of forcing it happens but too 

 often that the produce of the hot-house is totally 

 destroyed by the unskilful application of heat ; and 

 if heat is actually the cause of the sap's ascent, 

 how comes it that the degree necessary to produce 

 the effect is so very variable even in the same cli- 

 mate. For there are many plants, such as the 

 Arbutus, Laurus Tinus, and the Mosses, that will 

 continue not only to vegetate, but to protrude their 

 * Veg. Stat. Exper, 36. 



VOL, II. K 



