On the Heat of Forcing-houses. 131 



canes are so far from being injured by this sudden decrease 

 of temperature, that the sugars of Jamaica take a higher price 

 in the market, than those of the less elevated islands, of which 

 the temperature of the day and night is subject to much less 

 variation. 



During the progress of germination, in the spring, great 

 chemical changes take place in the component parts of the 

 sap of trees, analogous to those which have been observed in 

 the germination of seeds. T could not detect any vestige of 

 saccharine matter in the Alburnum, either of the stem or 

 roots, of the Sycamore tree in the winter ; but in the spring, 

 its sap became very sensibly sweet : and I found this sap to 

 be much more saccharine, and of greater specific gravity, in 

 large trees, which were prepared to nourish an abundant 

 blossom, than in small and young trees. The sap of the same 

 tree proved also to be subject to some variations of specific 

 gravity, at the same period of the spring, in different years ; 

 and Duhamel has observed, that the sap of the Sugar Maple 

 becomes first saccharine, and afterwards acquires an herba- 

 ceous taste ; in the latter state, it probably is best calculated 

 to feed the blossoms and unfolded buds. 



At the period of the preceding chemical changes in the 

 qualities and properties of the sap, previous to the growth of 

 the leaves, that fluid is found to ascend during the warm part 

 of the day, and to flow, in many species of trees, from any 

 recent wound, and to fall again during the night, particularly 

 if that be cold ; and as variations of temperature are the 

 apparent cause of these motions, it appears not improbable, 

 that the chemical changes, which take place in it at this 

 period, are promoted by the same agents. 



