Th£ vegetable system. 
11 
ferve their power of growth only fo long as their is vegetative life in 
them : and vegetable life confifts in a motion of the Juices. 
CHAP. VII. 
Of the Fall and Permanency of the Leaf. 
T H E Fall or fading of the Leaves at Autumn, or tlieir reten- 
tion all Winter in life and vigour on the Tree, affords aj great, 
an obvious, and a valuable diftindion : yet it is not fo certain as has 
been fuppofed ; nor can it be accounted for fo eafily. Hypothefes 
without proofs in nature folve all problems quickly ; but the advances 
made by obfervation are flow. They have however their reward ; 
they are eternal. 
Distinctions have been eftablifhed on the charadlers of the Ever- 
green and deciduous-leaved Trees : but fl;ri(fter obfervation fhews that 
the fame Tree will in one climate drop its Leaves in Autumn, which 
in another holds them all the Winter. The country where the Leaves 
are kept thro’ Winter is always warmer than that wherein the fame 
Tree lofes them in Autumn : and this leads us one ffep toward the 
general caufe v/hy fome Trees lofe and others hold them. Warmth 
gives rife to the fap, and the greater the degree of warmth is the 
more fap rifes: but we find there are In the fame climate fome Trees 
which hold and others which drop their Leaves ; though the warmth 
be equal. The Box and Holly are green with us all Winter ; while 
the Sallow and Hawthorn, and the generality of others, lofe their 
Leaves at Autumn : therefore it is evident, though the degree of heat 
be a part of the caufe, we mufi; feek further for the whole. Perhaps 
the error hitherto has been the attributing to one principle what was 
the refult of two or more. 
The caufe why Trees lofe their Leaves with us at Autumn is evi- 
dently the fame that makes Plants lofe their Stalks and die down to 
the ground : and this is the want of heat to raife the Juices to them : 
but we have feen that fome Sap rifes in all Trees in Winter ; and if 
we 
