352 



CONDUCTING SYSTEM 



become differentiated centripetally from a corresponding number of 

 peripheral points of origin, it is customary to discriminate between 

 diarch, tetrarch, pentarch, hexarch and polyarch bundles. The radial 

 hadrome-plates may meet at the centre of the bundle, in which case 

 they often converge towards a single specially wide axile vessel ; this 

 condition is exemplified by the primary roots of a number of Grasses 

 and of certain other Monocotyledons (Fig. 141). In other cases the 

 central region of the bundle is occupied by a parenchymatous pith 

 (Fig. 14 2-) or by a mass of fibrous tissue. 



P 





Fig. 141. 



Radial bundle of the root of Allium Ascalonieum, in T.S. ; g, large central vessel, 

 towards which the five hadrome-plates converge ; p, pericyele ; s, endodermis ; 

 d, passage-cells, situated opposite the hadrome-plates. 



Since roots, in which the radial type of bundle prevails, are 

 normally constructed as inextensible organs, the mechanical elements 

 are often distributed over the different vascular " rays " ; there is, in 

 fact, often no other way in which they can take up the central position 

 prescribed by the mechanical principle. Semilunar fibrous sheaths on 

 the outer side of the leptome-strands have only been observed in the 

 roots of certain Papilionaceae (Pisum, Phaseolus). The whole radial 

 bundle is surrounded by a pericanibiurn or pericyele, usually consisting 

 of a single layer of thin-walled parenchyma, which is chiefly concerned 

 with the production of lateral roots. In the Gkamineae, Cyperaceae and 

 Juncaceae, and in certain allied families, the pericyele is interrupted 

 in the majority of investigated species opposite the hadrome-plates, 

 which thus abut directly against the endodermis. 



