THE FIBRO-VASCULAR BUNDLES. 
119 
arranged; when completely developed they are thin- walled, with simple closed pits. 
Their contents in winter consist of starch, often associated with chlorophyll, tannin, 
and crystals of calcium oxalate. It also happens sometimes that the cambium-cells on 
the xylem-side of the bundle become transformed, without transverse division, into 
parenchymatous, thin-walled, simply pitted, elongated, succulent cells, which must 
also be considered as parenchymatous forms of wood-cells ^. To this last type are also 
to be referred the parenchymatous elements in the xylem-portion of the closed fibro- 
vascular bundles of Monocotyledons and Cryptogams; but these thin-walled, mostly 
elongated cells do not in this case originate in the cambium (since this, according 
to the terms in customary use, is absent from the closed bundles), but immediately 
from the procambium of the bundle (Fig. 95, p. 114, near S). Sometimes the wood- 
parenchyma derived from the cambium of Dicotyledons (parenchyma of the xylem) 
is more strongly developed, while only a few vessels and tracheides are formed : 
this occurs in the thick napiform roots of the radish, carrot, beet, and dahlia, and 
in potato-tubers. The apparent pith of these organs corresponds, in its origin, 
to the wood of a dicotyledonous tree ; but the elements of the xylem are not, or 
only slightly, lignified ; the succulent contents and the thin soft cell-walls scarcely 
give this xylem the appearance of a homologue of the ordinary wood, although there 
can be no doubt about the homology. 
The Phloem-portion of the fibro-vascular bundles shows, when fully dev_>!o| cd, 
similar cell-forms to the xylem-portion ; the sieve-tubes correspond to the vessels, 
the true bast-cells to the libriform fibres, the bast- or phloem-parenchyma to the 
wood-parenchyma. When the bast-parenchyma consists of long, narrow, very thin- 
walled cells, it has been termed by Nägeli Cambiform tissue. 
Parenchyma, cambiform tissue, and sieve-tubes may be included in the term Soft- 
bast^ in opposition to the true bast which is sometimes entirely absent (as in Cucurbita), 
but in other cases is very abundantly developed (as in the Jerusalem artichoke, lime, 
&c.), and consists of elongated, prosenchymatous, flexible, tough, firm cells, usually with 
strongly thickened walls. In Dicotyledons they are generally arranged in bundles, fre- 
quently forming layers alternating with soft-bast, as in the grape-vine ; but sometimes, 
especially in the later portions of the phloem, which are formed from the cambium, 
they occur also as separate fibres, as in the stem and tuber of the potato. The 
middle lamella of the partition-wall of two fibres is generally lignified or cuticularised 
(resistant and coloured yellow by iodine) when they are closely crowded ; but in 
other cases it forms a mucilaginous ' intercellular substance ' in which the cells (in 
transverse section) appear imbedded {e.g. the laburnum according to Sanio, Goniferae 
and especially in Cryptogams, see Fig. 95, h, p. 114). The true bast-fibres of the phloem, 
like the libriform fibres of the xylem, may become partitioned by subsequent septa 
(as in the vine, occidental plane, horse-chestnut. Pelargonium roseum, Taynarix gallica, 
according to Sanio, /. c. p. in). As the libriform fibres of the xylem are often found 
branched after isolation by maceration, so also are the bast-fibres ^. 
The forms of cells now described are the ordinary and essential constituents of the 
fibro-vascular bundles ; various other forms of tissue occur, however, occasionally, as 
' Sanio ai^plies to these cells the term ' Ersatzzellen.' 
^ It may not be superfluous to point out that many writers very inconveniently also designate 
certain cell-forms of the fundamental tissue as bast, when they are thick-walled, elongated, pointed at 
the ends, or even branched. In that case the libriform fibres of the xylem must also be called bast ; 
and it is evident that the term would then have no exact scientific meaning. The term bast-cells 
has, till recently, been given to hypodermal prosenchyma, to the bundle-sheaths of Grasses, 
Aroidese, and Palms, as well as to the cells which we have, in Sect. 14, called trichoblasts, even 
when they contain latex, as the laticiferous cells of Euphorbiaceee. This practice is greatly to 
be deprecated, as it must import great uncertainty into a correct interpretation of the different 
forms and systems of tissue. 
