ITBEOVASCULAR BUNDLES 



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Fibrovascular Bundles are groups of fibres, vessels and cells cours- 

 ing through the various organs of a plant and serving for conduction 

 and support. According to the relative 

 structural arrangement of their xylem 

 and phloem masses they may be classed 

 as follows: 



I. Closed Collateral, consisting 

 of a mass of xylem lying alongside of a 

 mass of phloem, the xylem facing to- 

 ward the centre, the phloem facing to- 

 ward the exterior. Stems of most 

 Monocotyledons and Horsetails. 



II. Open Collateral, consisting of 

 a mass of xylem facing toward the pith 

 and a mass of phloem facing toward the 

 exterior and separated from each other 

 by a cambium. Stems and leaves of 

 Dicotyledons and roots of Dicotyls and 

 Gymnosperms of secondary growth. 



III. BicoLLATERAL, characterized 

 by a xylem mass being between an in- 

 ner and an outer phloem mass. There 

 are two layers of cambium cells, one be- 

 tween the xylem and inner phloem mass, 

 the other between the xylem and outer 

 phloem mass. Seen chiefly in stems of 

 the Cucurbitacecs. 



IV. Concentric, characterized by 

 a central xylem mass surrounded by a 

 phloem mass or vice versa. No cam- 

 bium present. 



Fig. II.- 



-Stages in the devel- 

 opment of sieve tubes, companion 

 cells, and phloem parenchyma. 

 A, a and 6, Two rows of procam- 

 bial ceUs; in and d, a has divided 

 longitudinally and c is to become 

 companion cells; d, a sieve tube, 

 and b, phloem parenchyma. B, c, 

 Companion cells, and d, a beginning 

 sieve tube from c and d, respectively 

 in A. The cross-walls in d are 

 pitted; b, phloem parenchyma 

 grown larger than m A. C, The 

 same as B with the pits in the cross- 

 walls of the sieve tubes become per- 

 forations, and the nuclei gone from 

 the cells composing the tube. 

 (From Stevens.) 



later stage with cells longer and more pointed and walls becoming thickened and 

 pitted; 4, complete wood fibers with walls more thickened than in the previous stage 

 and lignified, as shown by the stippling. The protoplasts in this last stage have 

 disappeared and the fibers are dead. D, steps in the formation of wood parenchyma 

 from cambial or procambial cells, i, Group of cambrial or procambial cells; 2, 

 the same enlarged in all dimensions; 3, the same with walls thickened and pitted; 

 4 and 5 show the same stages as 2 and 3, but here the cells have enlarged radially 

 or tangentially more than they have vertically. The walls of these cells are apt to 

 become lignified, but the cells are longer lived than the wood fibers. {From 

 Stevens.) 



