23 
MORPHOLOGY OF TISSUES. 
lamina ; in the petiole they are generally surrounded by a thin-walled parenchymatous 
fundamental tissue with wide elongated cells ; this also forms sheath-like envelopes 
around the stronger bundles of the lamina, which are conspicuous on its under-side 
as the Feins; but the finer branches, and the finest of all, run through the so- 
called Mesophyll, z, e. a peculiar form of the fundamental tissue distinguished 
by containing chlorophyll. Not 
unfrequently single cells of 
the fundamental tissue of the 
lamina assume the form of idio- 
blasts, e.g. the larger stellate 
cells in the leaf of Camellia ja- 
ponica, the erect rod-like -cells 
upon which the stomata of the 
leaves of Hakea are, as it were, 
supported. All these forms of 
tissue are enveloped by the 
epidermis, and frequently also 
by hypodermal tissue. In the 
carpels of Phanerogams there 
occurs commonly a more mani- 
fold differentiation of the funda- 
mental tissue ; I will instance only 
the formation of the so-called 
' stones ' of Amygdalese. The 
stone is the inner layer of the 
fundamental tissue of the same 
foliar structure of which the outer 
layers form the succulent flesh of 
the fruit ; the former is scleren- 
chymatous, the latter parenchy- 
matous and succulent, both being 
traversed by fibro-vascular bun- 
dles. Equally clear is the struc- 
ture in the stems of Ferns, among 
which Tree - ferns and Pteris 
aquilina are of special interest, 
because the fundamental tissue 
occurs in them in two quite 
different forms. Its preponder- 
ating mass consists, e.g. in Pteris 
aquilina (Fig. 91, p. 109) of a 
thin-walled colourless mucila- 
ginous parenchyma, and con- 
taining starch in winter ; parallel 
to the fibro-vascular bundles 
there are also threads or 
bands of prosenchymatous thick- 
walled dark brown scleren- 
chyma; these have nothing in 
common with the fibro-vascular bundles, but are only a peculiar form of the funda- 
mental tissue, a prosenchymatous form of which often occurs also elsewhere in 
Cryptogams. The tendency to a prosenchymatous development of the cells of the 
fundamental tissue is also well seen in the stems of Lycopodiacese. In Selaginella 
denticulata (Fig. 100 A) the axial fibro-vascular bundle is surrounded by a very 
FIG. 100. — A transverse section of the stein of Selaginella denticulata; the 
fibro-vascular bundle is not yet fully developed ; the vessels are already ligni- 
fied on both sides, but not yet in the centre ; / air-conducting intercellular 
spaces in the parenchyma enveloping the bundle ; towards b is the tissue 
corresponding to the bundle which bends outwards to the leaf. B transverse 
section of the mature stem of Lycopodiujn Chajncecyparissus, the axial cylin- 
der consists of densely crowded and coalescent fibro-vascular bundles ; the 
four xylem-portions are quite separated, forming four bands on the transverse 
section, between and round which are found the narrower cells of the phloem. 
The phloem portions of the four bundles have coalesced; between each pair 
of xylem-bundles is seen a row of wider cells, the latticed cells or sieve-tubes ; 
the narrow cells lying on the right and left edge of each xylem-portion are 
spiral vessels (also in A). In the thick-walled prosenchymatous fundamental 
tissue which envelopes the axial cylinder is seen the dark transverse section 
of a slender fibro-vascular bundle which bends outwards to a leaf; it consists 
almost exclusively of long spiral vessels (x about 90). 
