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the relative arrangement of the ultimate vesicles 
of which it is composed. “In aged trunks,” says 
Keith, “ the outer layers are coarse and loose in 
their texture, exhibiting individually a consider- 
ably indurated, but very irregular network, com- 
posed of bundles of longitudinal fibres, not as- 
cending the stem directly, but winding more or 
less around the axis of the plant. As the layers 
recede from the circumference, the network 
which they form is finer, though still very irre- 
gular, and their texture more compact. Yet the 
meshes of the several layers often correspond, 
and form, at least in aged trunks, pyramidal 
apertures, or widen into large gaps or chinks, as 
in the trunk of the oak or elm, exhibiting still 
the rough traces of the original network. In 
young trees or shoots, the apertures formed by 
the coincidence of the meshes are not yet left 
empty, but are occupied by a pulp somewhat 
compressed, which traverses the longitudinal 
fibres, and binds and cements them together.” 
The liber, however, according to the use of the 
word by some phytologists, comprises only the 
innermost layer of a plant of any age, and, in 
the same manner as the alburnum, exists during 
only one year, or till thrust out of its position by 
the next annual deposit ; and this innermost 
layer, whether permitted to monopolize the 
name of liber or not, is always the finest and 
most delicate of the cortical layers, soft, smooth, 
flexible, capable of being reduced or subdivided 
into an absolute film, and often though not always 
exhibiting the most beautiful reticulation. 
The parenchyma consists of hexagonal cells con- 
taining a green-coloured juice in the branches and 
main body of the stem, and a colourless juice in 
the root and the portion of the stem beneath the 
surface of the soil; and even when the paren- 
chyma is covered by a thick and indurated epider- 
mis, the greenness of it in the parts above ground 
exists, and may easily be seen in any spot by the 
removal of a small portion of the epidermis. 
Plate VI. Fig. 1, exhibits the regular structure 
of the parenchyma or cellular substance, as seen 
under a powerful microscope. A transverse sec- 
tion of the hexagonal cells is seen in a, a verti- 
cal section of the same in 6, That the sides of 
the cells are common to those that are adjacent, 
appears from ¢. In J/g. 2a portion of cellular 
substance, with more porous membranes, and 
more elongated cells, is shown. F%g. 3 is an ex- 
ample of lengthened cells, pierced by pores in 
regular order. Jig. 4 is another specimen of 
cellular substance containing fissures as well as 
pores. In the early period of vegetable life, the 
cellular network-forms a number of lacunee, 
which, when matured, become a series of vessels, 
which in monocotyledonous plants are always 
found in the middle of woody fibres, or compose 
the greater part of them ; while in dicotyledonous 
plants, they appear to be disposed at random 
throughout the wood. Mirbel describes four 
kinds of these tubes : viz., 1st, The simple or con- 
BARK. 
329 
tinuous tubes, which commonly contain those re- 
sinous and oily juices which are known as the 
‘ proper juices’ of a plant. This class of pores is 
very distinct in the Euphorbie. They are shown 
in Fig. 5, Plate VI, 2d, The porous tube, the 
coats of which are penetrated by small holes in 
regular series. It is found chiefly in hard woods. 
See F%g. 6. 3d, The false air-vessels, which are | 
tubes transversely cut by parallel fissures. See 
Fig, 7. They are numerous in the vine. 4th, 
Air-vessels, chiefly found around the pith, and 
formed by the turnings of fibres from right to 
left. ig. 8 represents true spiral air-vessels of 
different sizes; (1g. 9, a large tube, with some 
of the spiral turns untwisted. 2g. 10 represents 
the whole of these organs united. 
On young shoots and stems, the epidermis ap- 
pears as if it were a thin transparent membrane 
without any vascular formation, yet, when exam- 
ined through very powerful glasses, it is seen to 
possess minute retiform vessels. J/g. 11,in Plate | 
VI, represents a portion of imperforate cuticle ; 
Fig. 12, a portion with elongated pores. In fig. 
13 a portion of ‘ velvety cuticle’ is represented; 
in which the magnified elevated points are 
merely an external elongation of the surface of 
the cellular membranes, or a kind of protrusion 
formed by that substance. Both the paren- 
chyma and the epidermis, however, belong rather 
to the young and soft parts of a plant than to 
the stems and older branches of trees and shrubs; 
and whenever ligneous plants pass the period of 
youth, the bark of their stem loses all paren- 
chyma and epidermis, and begins to convert even 
the outer layers of the liber into dead matter, 
and to commence a constant series of effort, to 
throw them away. The thin transparent skin 
which constitutes the epidermis is ruptured, and 
never afterwards renewed; the green matter of 
the parenchyma, after exposure to the air, be- 
comes brown, and loses its vitality; the outer- 
most layers of the liber harden, die, and crack ; 
an exterior, many-shaped, and even amorphous 
coating of dead or excrementitious matter is 
formed; and a constant struggle of the enclosed 
living plant goes on to free itself of this dead 
matter, either by the exfoliation of layer after 
layer as in the case of the birch, or by the rejec- 
tion of considerable plates or nodules as in the 
case of the plane-tree and the pine.—In such 
monocotyledonous plants as are strictly endo- 
genous, or effect their growth by simple en- 
largement of the centre of their stems, the bark 
undergoes no other change, from infancy to old 
age, than to lose its green colour, and become 
more fibrous; and in such monocotyledonous 
plants as are acrogenous, or effect their growth 
principally by elongation of their centre, it un- 
dergoes no other change than to become more | 
compact or indurated. 
The bark protects the enclosed wood and its 
own innermost layers from the effects of incle- 
mency and vicissitude in the weather ; and there- 
