90 THE LIBER OR PHLOEM. 
organically connected by means of the medullary rays and 
cambium-layer (jig. 176, c, c). When the stem is first formed 
the bark is entirely composed, like the pith, of parenchyma ; 
but as soon as the wood begins to be developed on the 
outside of the pith, certain cells which le nearer the surface 
of the stem make their appearance, which develop into liber- 
cells and certain vessels (jig. 190, d). Externally to these 
lie other parenchymatous cells, the inner ones of which form 
the green layer of the bark, c, whilst the outer cells become 
developed into the cork tissue, b, and these again are invested 
by colourless cells forming the epidermis, a, so that the 
Fra. 190. bark, when fully formed, 
consists of two distinct sys- 
tems ; namely, an imternal 
or fibro-vascular, and an ex- 
ternal or parenchymatous. 
Further, the parenchyma- 
tous system, as just noticed, 
also exhibits, in all plants 
which are destined to live 
for any period, a separation 
into two portions ; and the 
whole is covered externally 
by the epidermis already 
described (fig. 190, a). 
The fully developed bark 
accordingly presents three 
distinct layers, in addition 
Fig. 190. Transverse section of a portion of aes epidermis, which is 
the bark of an Exogenous stem, a. Epi- Common to it and the 
dermis. 6. Corky layer. c. Cellular en- other external parts of 
velope. d. Liber or Phloém. plants. The three layers 
proper to the bark are 
called, proceeding from within outwards : 1. Liber, Inner Bark, 
or Phloém (figs. 190, d, and 185, A, and B, f, f) ; 2. Cellular 
Envelope, Green Layer, or Phelloderm (figs. 190, c, and 185, 
g, 9); and 8. Corky Layer, Suberous Layer, or Outer Bark (figs. 
190, b, and 185, h, h). 
a. The Liber, Inner Bark, or Phloém (figs. 190, d, and 185, 
A, and B, f, f).—This is composed of true bast tissue, or, as it 
is also called, woody tissue of the ber, as it consists of narrow 
elongated cells with thickened and flexible walls ; mixed with 
parenchymatous (cambiform) tissue and sieve-tubes filled with 
albuminous matters, and frequently laticiferous vessels. The 
phloém therefore belongs to the fibro-vascular system, and forms 
the portion of the indefinite fibro-vascular bundles outside 
the cambium (page 76). The portion formed of bast tissue is 
sometimes termed hard bast, and the cambiform parenchyma and 
sieve tubes together constitute what is then called the soft bast. 
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