The vegetable SYSTEM. 
I z 
we would know why this quantity of Sap is enough to keep the 
Leaves alive in certain kinds, and not in others, we muft feek it in 
-thofe Juices, and their texture. 
The Juices of the Hawthorn are thin and watery ; thofe of the 
Holly are. thick even to a degree approaching to bird-lime : the Leaf 
of the Hawthorn is full of large pores for evaporation ; the Leaf of 
Holly has few and fmall : this holds in all the deciduous-leaved 
and ever-grecn trees in a greater or lefs degree : thefe are feledted only 
becaufe it is moll obvious in them. This gives the reafon of the dif- 
ference ; and explains why in a warmer climate the fame- fpecies may 
keep the Leaf which here lofe it. Leaves fall becaufe the fupply of 
Juices from the Root is not equal to the wafte by evaporation : and 
therefore thofe which perfpire or evaporate moft will fall firft, and 
thofe which lofe this way the lead: quantity of their Juices will laft 
longeft. The Leaves are kept on Trees by a due fupply of moifture 
from the Root : it is not that the Holly has more of this fupply than 
Plawthorn 3 but it lofes lefs : which in the end is juft thefame. 
Deciduous-leaved Trees become Evergreens in countries where 
the encreafed warmth of the air gives this fupply ; and in our own 
country the Holly and the like retain their Leaves, becaufe the fmali 
pores and the thickened nature of the Juice prevent evaporation. The 
Sap is watery when it is received at the Root 3 but by that time it 
reaches the Leaves it is aflimilated, and becomes of the nature of the 
Plant: therefore the tougher the nature of the Juices of the Plant are, 
the lefs fupply will anfwer 3 becaufe it is fo much the more fecure 
from lofs by evaporation. This and the clofe texture of the Leaves 
themftlves together give the quality of retaining the Leaves : and we 
fee this illuftrated by a moft plain example in grafted and inoculated 
Trees, where the ftock is a deciduous-leaved kind, and the graft an 
Ever-green. Many of the American Oaks are Evergreen, and 
when we raife them on the ftock of our own Oak, which is deciduous, 
they yet retain their Leaves all Winter. 
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