128 
MORPHOLOGY OF TISSUES. 
course of these bundles necessitates that they must cross one another, and form 
a loose mass consisting of slender isolated' strings, surrounded by a more or less 
thick layer of denser woody substance. This woody substance forms a cylinder 
which, in decayed stems, is altogether separated from the layer of periderm, and 
loosely enveloped by it. The isolated strings in the interior are the primary 
vascular bundles which have been formed during the growth in length (properly only 
their lower ends or Leaf-iraces^ since the upper ends bend outwards into the 
leaves). The woody cylinder which envelopes them all consists, on the contrary, of 
secondary fibro-vascular bundles formed by 
the increase in thickness, which are closely 
crowded and anastomose copiously with 
one another both in the tangential and 
radial directions, and thus form, accord- 
ing to circumstances, a more or less com- 
pact or spongy mass, the true nature of 
which it is easy to recognise in Aloe 
and Beaucarnea. The course of develop- 
ment of these stems is as follows. The 
isolated fibro-vascular bundles (which in 
old specimens are found in the interior) 
are formed in the primary meristem of the 
apex of the stem, while the whole remaining 
tissue between them passes over into pri- 
mary fundamental tissue ; but after con- 
siderable time (in Ale iris fragrans it takes 
place about 4 or 5 cm., in DraccBiia reflexa 
as much as 17 to 20 cm. below the apex 
of the stem) a fresh formation of (second- 
ary) meristem begins in one of the cell- 
layers of the fundamental tissue which 
immediately surround the outermost fibro- 
vascular bundles. The permanent cells 
concerned in it divide repeatedly by tangen- 
tial and subsequently sometimes by radial 
walls ; and there arises (seen in transverse 
section) a ring of meristem (Fig. 104, x), 
the cells of which are arranged in radial 
rows. In this meristem new fibro-vascular bundles are produced ; one, two, or more 
adjoining cells (on the transverse section) dividing repeatedly by longitudinal walls 
in various positions. Out of the procambium-bundles which arise in this manner 
the fibro-vascular bundles proceed immediately, the procambium-cells being trans- 
formed into fibro-vascular tissue ^ ; the intermediate meristem passes over likewise 
Fig. 104. — Part of the transverse section of a stem oi D>a- 
ca7ta (probably reflexa) about 13 mm. thick and i metre high, 
about 20 cm. below the summit, e epidermis ; k cork (peri- 
derm) ; r cortical portion of the fundamental tissue ; b transverse 
section of a fibro-vascular bundle, bending out to a leaf ; 7)i the 
primary fundamental tissue (pith) ; g the primary vascular 
bundles ; x the ring- of meristem in which very young fibro- 
vascular bundles are to be seen, while the older ones g have 
already partially or entirely passed out of it, its lower part 
becoming transformed into fundamental tissue st arranged 
in radial rows. 
^ It appears, however, that the thick-walled lignified cells on the outside of such a bundle 
do not belong to it, but to the secondary fundamental tissue, and therefore represent only a 
sclerenchymatous bundle-sheath, while the bundles enveloped by them are themselves very slender. . 
