238 Mr Murray on the Phenomena 
The experiment is a very simple one. Let a leaf be torn from 
the twig, and a portion of bark and wood follows. When the 
leaf assumes the autumnal tint, then indeed, as with the ash, &c. 
it may he insulated, but the leaf is now dead, the vessels of sup- 
ply no longer minister their living juices, -^its veins are exhaust- 
ed, and it hangs a withered appendage, and prey to the first 
autumnal breeze. 
There are, in like manner, circumstances which do not har- 
monize with the first opinion : but to mention the fall of the 
early blossom in an analogous relation, the sudden denudation 
of the mulberry-tree, without any change of colour, by early 
frost *, shews us that the cause may act suddenly and prema- 
turely, and must be something distinct from the dimensional 
increase of the hud. 
The sap has its periodical revolution, and as it moves in its 
channel, acted on and influenced by the vicissitudes of an atmo- 
sphere ever varying its density , and the thermometric , hygro- 
metric , and electrical relations of which are always fluctuating, 
it must change its pulse conformably ; and the flux of its stream 
must correspond with the circumstances on which it hinges, and 
by which it is accelerated or retarded, regulated or checked* 
The spring gives an animating stimulus to the punctum sa- 
liens of vegetation, which is thus aroused from its hyherna- 
culum, and the tree then assumes its beauteous and refreshing 
mantle of green. This is succeeded^ as the season advances, 
by the blossom and the fruit. 
Now, the sap must not only be much varied in its periodic 
times of revolution, but be obedient to the external agents enu- 
merated, and thus will the character of its deposits be essentially 
changed. The sap, at first rapid, becomes, as the sun advan- 
ces toward the tropic of Capricorn, more languid in its course. 
The tints of autumnal foliage are the external insignia which 
announce the changes that take place. The gradual disunion 
and final fall of the leaf, is connected with the gradual contrac- 
tion and close of the anastomosing vessels, and this constriction 
is the consequence of a loss of caloric, by reason of radiation 
to the unclouded sky; or, in the absence of the illuminating 
sunbeam, in dark weather. All circumstances being the same, 
* See Bradlev on Gardening, Lond. vol. i, p. 194., and vol. ii. p. 54. 
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