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CONDUCTING SYSTEM 



several concentric circles of bundles are developed (Marattiaceae, spp. 

 of Uteris), while in others the main vascular cylinder is supplemented 

 by small accessory medullary and cortical strands. It is probable that 

 these complications arise as a result of increased demands in respect of 

 translocation ; but the detailed investigations which might furnish 

 evidence in support of this suggestion, have yet to be carried out. 



3. Attention must next be directed to a third and very widely dis- 

 tributed type of vascular arrangement, which may be shortly entitled the 

 Dicotyledonous type. The characteristic features of this type, which 



occurs in the great majority of Dicotyledons, 

 and in the Coniferales and Gnetales, as well 

 as in a few Monocotyledons (Dioscoreaceae) 

 and Vascular Cryptogams (Equisetum and 

 the Osmundaceae), are three in number. 

 In the first place, all the primary vascular 

 strands are common bundles, which enter the 

 stem from the leaf along a curved path, and 

 afterwards run down through several inter- 

 nodes. Secondly, while a bundle is traversing 

 the stem in a vertical direction, it always 

 remains approximately at the same distance 

 from the centre. Lastly, each leaf-trace 

 bundle sooner or later attaches itself later- 

 ally, either with or without previous division, 

 to a neighbouring leaf-trace belonging to an 

 older leaf; the leaf-traces thus all combine to form a continuous system, 

 which is either unilaterally sympodial or reticulate in character. In 

 consequence of this behaviour of the vascular strands, a transverse 

 section of the stem reveals a single circle of bundles, surrounding the 

 pith and itself surrounded by the cortex, these two portions of the 

 ground-tissue being connected by the primary medullary rays which 

 occupy the radii between the bundles. It may be remarked in passing, 

 that these interfascicular strips should not be called medullary rays, in 

 the anatomico-physiological sense of the term, unless they actually 

 consist of parenchyma and thus constitute a histological link between 

 cortex and pith ; when, on the contrary, the vascular bundles are 

 embedded in an uninterrupted fibrous cyliuder, the interfasicular strips 

 of bast must not be called medullary rays, since a layer of mechanical 

 tissue forms just as effective a barrier between pith and cortex as the 

 vascular bundles themselves. 



We may next consider a few examples of the Dicotyledonous type 

 of vascular arrangement in greater detail. The adjoining figure (Fig. 

 151a) illustrates the course of the vascular bundles in a young twig of 



Fig. 150. 



Tubular vascular network (dictyo- 

 stele) in the distal portion of the 

 stock of Aspidhim Filix runs ; on the 

 right a single mesh of the network, 

 more highly magnified, to show the 

 insertion of the foliar strands. 



