THE FIDRO -VASCULAR BUNDLES. 



01 



with iodine-solution^. The cell-walls become dissolved, the protoplasmic mucilage 

 remains behind coloured brown, and may be recognised in the form of fine strings 

 of mucilage filling up the pores of the sieve-disc (Fig. 86, />). Those cells may pro- 

 visionally (after Von IMohl) be called latticed-cells in which similar formations of wall 

 are visible, even although the previous perforation of the narrow crowded pits 

 (lattice) cannot be proved. To this category belong the so-called * Vasa propria' 

 in the fibro-vascular bundle of Monocotyledons (Fig. 8i, nj), and the form of cells 

 discovered by Dippel in Cryptogams, and called by him bast-vessels. (Dippel, /. c.) 

 The latticed cells or sieve-tubes frequently have sieve- or latticed discs in their 

 longitudinal walls also, when two cells of this kind are placed in juxtaposition 

 side by side ; these discs are thinner portions of the cell-wall which show a fine 

 puncturing or lattice-like thickening ; whether in 

 these cases actual perforations also occur is still 

 undetermined. These cell-formations (cambiform, 

 latticed cells, sieve-tubes) may, in combination with 

 the phloem-parenchyma in which they are imbed- 

 ded, or which sometimes forms thicker layers, 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 (e. g. stem of Helianthiis tiiberosus, Tilia, 

 &c.), and consists of elongated, prosenchymatous, 

 fibre-like, flexible, tough, firm cells, usually greatly 

 thickened. In Dicotyledons they are generally ar- 

 ranged in bundles, frequently forming layers alter- 

 nating with soft-bast (as in the grape-vine) ; but 

 sometimes, especially in the later portions of the 

 phloem, which are formed by the cambium, they 

 occur also in 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 

 (not dissolving and turning yellow with 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, 

 Coniferae). The true bast-fibres of the phloem, like the fibres of the wood, may become 

 partitioned by subsequent septa (as in the vine, occidental plane, horse-chestnut, Pelar- 

 gonium roseiini, Tamarix gallic a, according to Sanio, I.e. p. iii). As the wood-cells are 

 often found branched after isolation by maceration, so also are the bast-fibres, which 

 frequently attain greater freedom at the expense of the surrounding soft tissue {Abies 

 pectinata, according to Schacht). Sometimes the bast-cells are short and lignified when 

 more decidedly thickened, and very hard (tuberous roots of Dahlia). In Apocynaceae 

 {e. g. Vinca) the very long bast-cells are alternately wider and narrower, and also dis- 

 tinctly striated (on laticiferous bast-cells 'vide infra). The true bast-cells of the Equi- 

 setaceae, Ferns, and Lycopodiaceae (found by Dippel) are but little developed, the external 

 thickening-layers of their walls being apparently generally mucilaginous ^ (or developed 

 as intercellular-substance). 



Fig. S6.— Combinations of sieve-tubes, showing 

 the perforation of the septa after solution of the 

 cell-wall by sulphuric acid. A and B from the 

 leaf-stalk of Cucurbita ; C from the stem of 

 Dahlia. At A the cell-wall h h' is not yet com- 

 pletely absorbed; s' the protoplasmic mucilage, 

 o and « accumulation of it on the upper and 

 under side of the septum ; / the strings of mu- 

 cilage, which unite these accumulations and fill 

 up the pores of the sieve-discs {cf. Figs. 23 and 

 24). 



^ Cf. Sachs, in Flora, p. 68, 1863, and other proofs of the perforation in Hanstein, Die Milch- 

 gefisse, Berlin, pp. 13 et seq., 1864. 



"^ There is no reason for describing, as is done by many writers, as bast the hypodermal fibres 

 of Equisetum, the Lrown-walled prosenchyma in the fundamental-tissue of the stem of Tree-ferns 

 and Pterin aqidlina, and other cell-formations which do not at all belong to the fibro-vascular 

 bundles. 



