f 



THE FIBRO-VASCULAR BUNDLES. II9 



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'lopfd, 

 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 Nageli Cambiform tissue. 



Parenchyma, cambiform tissue, and sieve-tubes may be included in the term Scft- 

 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, b, p. 1x4). 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, Tamarix gallica, 

 according to Sanio, /. c. p. iii). 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 applies to these cells the term 'Ersatzzellen,' 



^ It may not be supei-fluous 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 vi'ould then have no exact scientific meaning. The term bast-cells 

 has, till recently, been given to hyi^odermal prosencliyma, to the bundle-sheaths of Grasses, 

 Aroideoe, and Palms, as well as to the cells which we have, in Sect. 14, called trichohlasts, even 

 when they contain latex, as the laticiferous cells of Euphorbiaceoe. This practice is greatly to 

 be deprecated, as it must import great uncertainty into a correct interpretation of the different 

 fonns and systems of tissue. 



