THE FIBRO-VASCULAR BUNDLES. 
109 
tissue, which in some cases develop by increase in thickness in such a manner 
that they lose externally the form of strings and present that of large masses, 
retaining, however, internally their characteristic structure. These are the Vascular 
or Fibro-vascular Bundles. Very often they can be completely isolated with ease 
from the rest of the tissue of the plant. If, for instance, the petiole of Plantago 
major is broken across, they hang out from the parenchyma as tolerably thick, 
extensible, elastic threads. In Pteris aquili7ia it is possible, by scraping off the 
mucilaginous parenchyma, after removing the hard epidermal tissue of the under- 
ground stem, to expose them as strap-shaped or filiform very firm light yellowish 
bands (Fig. 91). In older leaves of trees, dry pericarps (as Datura), stems of 
Cactus, &c., the fibro-vascular bundles are left, through the decay of the parenchyma 
which surrounds them, as a skeleton retaining more or less the original form. 
Beautiful and instructive skeletons of this nature 
are afforded by the stems of Tree-ferns, Dra- 
ccEna, Yucca, maize, &c., when their parenchyma 
has been destroyed by gradual decay, and 
only the epidermal tissue and the firm bundles 
in the interior remain ; and the student would 
do well in any case to make for himself pre- 
parations of this kind, or to examine them in 
collections ; they are extremely useful for a clear 
comprehension of structure. This is, however, 
the case only with lignified fibro-vascular bundles 
which run isolated between soft parenchyma ; 
in some plants, on the contrary, the tissue of 
the bundles is even softer and more delicate 
than that which surrounds them {e.g. Cerato- 
phyllum, Myriophyllum, Hydrilleae, and other 
water-plants) ; and in these cases they cannot of 
course be isolated. But in the older lignified 
stems and roots of Conifers and Dicotyledons, 
the fibro-vascular bundles are so densely crowd- 
ed, and so extended by the further development 
of their tissue, that at last very little or even 
nothing is left of the original fundamental dssue which separated them, and such 
stems consist almost entirely of fibro-vascular masses. 
Each separate fibro-vascular bundle consists, when it is sufficiently developed, of 
several different forms of tissue, and must therefore itself be considered as a tissue- 
system ; but different bundles, often in very large number, unite in most plants to 
form a system of a higher order. At present however we shall consider only the 
separate bundle. 
The fibro-vascular bundle consists at first of similar cells fitting together 
without intercellular spaces ^ ; this form of tissue in the young undifferentiated 
^ The young cells of the fibro-vascular masses are not always elongated and prosenchynaatous ; 
in the roots of maize the young vascular ctUs which no longer divide, as well as the adjoining ones, 
are tabular or cubical. 
Fig. 91. — Fteris aquiliiia. A transverse section 
of the untlerground stem (natural size) ; r brown hard 
epidermal tissue ; / soft mucilag nous parenchyma 
containing starch ; pr dark-walled sclerenchyma, form- 
ing two broad bands traversing the stem ; ag fibro- 
vascular bundles running outside these bands of 
sclerenchyma ; ig others running within them. B the 
fibro-vascular bundle represented in A, isolated by 
scraping off tlie parenchyma; it exhibits divisions 
and anastomoses; the dotted lines ii show ihe outline 
of the stem j , of its branches st' and st" , and of a 
petiole b. 
