214 ON THE HEAT IN FORCING-HOUSES. 



low during the night. In Jamaica, and other mountainous islands of 

 the West Indies, the air upon the mountains becomes, soon after sunset, 

 chilled and condensed, and, in consequence of its superior gravity, 

 descends and displaces the warm air of the valleys ; yet the sugar-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. I 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 herbaceous 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. 



Some experiments which I have made upon germinating seeds, have 

 perfectly satisfied me, that these afford plants of greater or less vigour in 

 proportion as external circumstances are favourable in promoting beneath 

 the soil the necessary changes in the nutritive matter they contain ; and 

 I suspect that a large portion of the blossoms of the cherry and other 

 fruit-trees in the forcing-house often proves abortive, because they are 

 forced, by too high and uniform a temperature, to expand before the sap 

 of the tree is properly prepared to nourish them. 



I have therefore been led, during the last three years, to try the effects 

 of keeping up a much higher temperature in the day than in the night ; 



