DIV. I 



MORPHOLOGY 



137 



and that in Fig. 163 pentarch. The vascular strands may either meet 

 in the centre (Figs. 161, 163) or there is in this position a central 

 strand composed of parenchyma 

 or sclerenchyma or a mixture 

 of these tissues (Fig. 160). 

 Most roots have to be con- 

 structed to resist pulling strains, 

 and the mechanical tissue is 

 accordingly mainly placed com- 

 pactly in the central pith (Fig. 

 162). 



For an organ that ^ias to resist 

 tension it is immaterial at what part 

 of the cross-section the mechanical 

 tissues are placed. Their association 



in the centre to form a single strand is of advantage, since, if many thinner strands 

 were situated peripherally, a one-sided pull might rupture some of these more 

 readily. 



The continuity of the xyleni and phloem strands of the radial bundle of 



FIG. 162. Mechanical tissue of roots. 1, Centrally 

 placed to resist longitudinal pulling strains ; 2, a 

 prop root with a peripheral layer of mechanical 

 tissue (P) to resist lateral pressure, in addition 

 to the central strand. (After XOLL.) 





FIG. 163. Transverse section of the radial bundle of the root of Allium ascalonicum. s, 

 Endoder mis with the inner walls thickened ; d, transfusion cells ; p, pericycle; g, large central 

 1. (ROTHERT after HABERLANDT.) 



the root with the corresponding tissues of the differently -constructed bundles 

 of the stem is effected at the junction of the root and stem of the seedling. 

 It need only be briefly described for the most common case of plants in 

 which the bundles of the stem are collateral. The essential fact of the transition 



