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Sefides the capillary tubes, &c. Ibefore-mentiohed, 
there are other larger veffels, which fqme call venal 
veffels, which are difpofed on the outfide of the arte- 
rial veffels, between the wood and the inner bark, and 
lead down to the covering of the inward root. Thefe 
veffels are fuppofed to contain the liquid fap found in 
plants in the fpring, &c. 
The Rev. Dr. Hales tells us in his excellent treatife 
on Vegetable Statics, that, in order to find whether 
there was any lateral communication of the fap and 
fap-veffels, as there is of the blood in animals, by 
means of the ramifications and lateral communica- 
tions of them veffels •, he took a young oak-branch, 
feven or eight inches diameter, at its tranfverfe cut, fix 
feet high, and full of leaves ; and having cut a large 
gap to the pith, feven inches from the bottom, and 
of an equal depth the whole length, and alfo cut an- 
other gap four inches above that on the oppofite fide, 
he fet the end of the ftem in water, and in two nights 
and two days time it imbibed and perfpired thirteen 
ounces; while another like oak-branch, fomewhat 
bigger than that, but with no notches cut in its ftem, 
imbibed twenty-five ounces. 
From this and many other experiments he there men- 
tions, he fays, we fee a moft free lateral communi- 
cation of the fap and fap-veffels, thofe great quanti- 
ties of liquor having paffed laterally by the gaps ; in 
that, by feveral experiments on cylinders of wood, 
little evaporated by. the gaps. 
The bark is the exterior part of trees, ferving them 
for a fkin or covering : it is generally of a fpongy tex- 
ture, and communicates with the pith by a multipli- 
city of fmall fibres paffmg through the capillary tubes, 
of which the wood confifts : fo that the roots having 
imbibed the proper nutriment of the tree, it is carried 
up by the warmth of the fun, through the fine arte- 
rial veffel of the tree, to the top of it •, and being 
there condenled by the cold, it does, by its own gra- 
vity, return down by the veffels which lie between 
the wood and the inner bark, which perform the of- 
fice of veins •, and as it pafles by, leaves fuch parts 
of its juice as the texture of the bark will receive, 
and requires for its fupport. 
Some are of opinion, that that foft whitilh rind, or 
fubftance, which lies between the inner bark and the 
wood, does the office of veins : and fome call this a 
third bark, and fuppofe it to differ from the other in 
nothing but having clofer fibres, and that it contains 
the liquid fap, gums, &c. which are found in the 
plants in the fpring and fummer months, which har- 
dens by degrees, by means of the fap it tranfmits, and 
is imperceptibly conveyed into the woody part of the 
'tree. 
The bark ferves for divers purpofes for it not only 
tranfmits the nutritious juices of the plants, but alfo 
contains divers fat oily humours, to defend the inner 
parts from the injuries of the weather. As animals 
are furnifhedwith a panniculus adipolus, ulually re- 
plete with fat, which invefts and covers all the flefby 
parts, and fereens them from external cold ; lo are 
plants encompaffed with a bark, replete with fat juices, 
by means whereof the cold is kept out, and, in win- 
ter-time, the fpicuise of ice prevented from fixing 
and freezing the juices in their veffels ; whence it is, 
that fome forts of trees remain ever-green throughout 
the year, by reafon their barks are more compadt, 
and contain a larger quantity of oil than can be ipent 
and exhaled by the fun. 
The pith is the inward central part of a tree or plant, 
anfwering to the medulla, or marrow of an animal. 
As for its fubftance, it confifts of little transparent 
globules, ' chained or linked together, fomewhat like 
the bubbles that compofe the froth of liquor. 
Some fuppofe, that the circulation of the fap is ef- 
feded by means of the pith, others by the bark, and 
others by the wood. 
Borelli, in his book De Motu Animalium, fuppofes 
the tender growing (hoot to be diftended like foft wax, 
by the expanfion of the moifture in the fpongy pith ; 
which dilating moifture, he concludes, is hindered 
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from returning back, while it expands by the fpongb 
nefs of the pith without the help of valves. 
And the Rev. Dr. Hales is of opinion, that it is very 
probable, that the particles of water that immediately 
adhere to, and a re ftrongly imbibed into, and attrad- 
ed by, every fibre of the fpongy pith, will fuffer fome 
degree of expanfion before they can be detached by 
the warmth of the fun from each attrading fibre ; and, 
confequently, the mafs of fpongy fibres, of which the 
pith confifts, muft therefore be extended. 
And that the pith may be the more ferviceable for 
this purpofe, nature has provided in moft {hoots a 
ftrong partition at every knot, which partitions ferve 
not only as plinths or abutments for the dilating pith 
to exert its force on, but alfo to prevent the too free 
retreat of the rarefied fap from the pith. 
But a dilating fpongy fubftance, by equally expand- 
ing itfelf every way, would not produce an oblong 
fhoot, but rather a globous one, like an apple ; to 
prevent which inconvenience it is obfervable, that 
nature has provided feveral diaphragms, befides thofe 
at each knot, which are placed at fmall diftances acrofs 
the pith, thereby preventing its too great lateral dila- 
tation. 
Thefe are very plain to be feen in the fhoots of the 
Walnut-tree, and the fame may be obferved in the 
pith of the branches of the Sun-flower, and feveral 
other plants, where thofe diaphragms are not to be 
diftinguifhed while the pith is full and replete with 
moifture ; yet when it dries up, they are often plain 
to be feen. And it is farther to be obferved, that 
where the pith confifts of diftindl veficles, the fibres 
of thofe veficles are often found to run horizontally, 
whereby they can the better refill: the too great lateral 
relaxation of the root. 
The trunk and branches of a tree, bear a refemblance 
to the exterior members and limbs of an animal, 
which it may fubfift without, though the rotting and 
mortification of them oftentimes occafion a total de- 
ltrucftion of it. Accordingly the like eftedts are found 
from the wounding or lopping of a tree, as from the 
wounding or cutting off a limb, an extravafation, cal- 
lus, or the like. 
A leaf is part of a plant extended into length and 
breadth in fuch a manner, as to have one fide diftin- 
guiihable from the other. The leaves, according to 
Malpighius, confift of fo many interwoven utricles, 
as to be not much unlike a pulmonary net, and ferve 
inftead of lungs to the plant. As the perfpiration 
and refpiration are chiefly performed thereby, thofe 
veffels are very confpicuous when the leaves are dif- 
fered. In the day-time, when the heat hath rarefied 
the mounting juices, fo as to become fpecifically 
lighter than the air, they flow out through the pores 
of the leaves, and evaporate, which is the occafion of 
the leaves becoming fo flaccid in very hot weather ; 
but in the night, when by the cold the juices are 
more condenfed, then the leaves are eredted again, and 
draw in a great fhare of nourifliment from the air. 
Thefe leaves we may obferve to be of different tex- 
tures on each fide, the upper flirface being for the 
moft part fmooih, the better to fhoot off the redun- 
dant moifture, while the under furface is many times 
of a rough and cottony texture, by which it is capa- 
ble of retaining the moifture ; for which reafon we 
find, if by bad management, &c. the fhoots of trees 
are nailed to a wall, &c. fo as to turn the flirfaces of 
the leaves the wrong fide upwards, the fhoots will be 
at a Hand, until the leaves have obtained their proper 
difpofition. Thefe leaves, as the learned Dr. Hales 
obferves, are carefully diftributed at fmall diftances 
throughout the whole length of the fhoots, and ferve 
as fo many jointly- adting powers, placed at different 
ftations, thereby with more eafe to draw plenty of fap 
to the extending root, 
A flower is the more tender part of a plant, remark- 
able for its colour, or form, or both, cohering with 
the rudiment of the fruit, and contains the organs of 
generation ; fome of thefe flowers contain the male 
organs, as the ftamina and apices, which are loaded 
7 with 
