386 PRIMARF ARRANGEMENT OF TISSUES. 



course of the bundle shows that the primitive vessels extend continuously from the 

 bundles of the stem into that of the root, and thus gradually become displaced, on their 

 way to the root, from the central into the peripheral position. This is accompanied 

 by a corresponding gradual displacement of the other parts of the bundle, in such a 

 direction that, on their union to form the root-bundle, the radial structure of the latter 

 is also attained. Together with these displacements, division and re-union and 

 disappearance of certain parts of the bundle may successively take place. It is 

 obvious that a variety of particular cases are possible according to the dififerent 

 number of the bundles in the hypocotyledonary stem, and of the rays in the bundle 

 of the root. 



Among the particular cases which have been more accurately investigated, that described 

 by Strasburger in the case of Biota orientalis, which presumably often recurs elsewhere, is 

 especially simple and intelligible. The hypocotyledonary stem contains in its upper part 

 two bundles descending perpendicularly from the two cotyledons ; the bundle of the main 

 root is diametrally diarch. In each of the cotyledonary bundles as they gradually ap- 

 proach each other a radial division of the phloem into two halves begins. Further below 

 the two halves become more and more distant, and are shifted into the same tangential 

 plane with the xylems ; each then approaches that coming from the same side of the other 

 bundle and unites with the latter to form one broad phloem-group. The xylem-groups of 

 both bundles go through the above-mentioned displacement or torsion in the same part 

 of their course ; the two broad phloem-groups therefore alternate with the xylem-plates 

 of the root-bundle, which have their primitive vessels directed towards the outside. 



In the Mietineas with many cotyledonary bundles and a polyarch bundle in the main 

 root (cf. p. 356), as many cotyledonary bundles as the bundle of the root has xylem-plates 

 undergo torsion during their downward course through the hypocotyledonary stem; their 

 phloem-groups shift laterally into the space between the xylem-groups, the primitive 

 vessels of the latter shift from the central to the peripheral edge. The xylem-groups 

 of the remaining cotyledonary bundles gradually disappear, while the phloem-groups 

 amalgamate with those which undergo the deviation described. 



In Phaseolus four decussate pairs of bundles traverse, in the simplest case, the hypo- 

 cotyledonary stem. The xylem-groups of the eight bundles are separate, the phloem- 

 groups are united to form four broad curved bands, which together form a ring only 

 interrupted between the two bundles of each pair. The xylem-groups of the bundles 

 leave the cotyledons with normal orientation, but then go on changing their direction in 

 such a manner that the primitive vessels of each come to stand in one tangential row witli 

 the others ; and in fact the bundles of each pair here turn their primitive vessels towards 

 each other. At the boundary of the main root the xylem-groups of each pair revolve 

 round the primitive vessels in such a manner that the latter come to lie outside, the re- 

 maining vessels towards the inside ; the further down, the more acute is the angle which 

 the bundles of a pair form with one another, until they finally become parallel, and fuse to 

 form one of the four xylem-plates of the root-bundle. The phloem-strands alternating 

 with the latter are the direct continuation of the four broad bands of the hypocotyle- 

 donary stem. Both in the latter and in the root they are each supported externally by a 

 thick strand of selerenchymatous fibres, which however is interrupted for a short distance 

 at the point of transition to the root. In other less simple cases, additional intermediate 

 pairs of bundles lie between the above-mentioned four pairs of bundles of the hypocotyle- 

 donary stem. These usually end blind at the lower boundary of the stem ; one of them 

 however frequently enters the root, so as to form a fifth xylem-plate of the pentarch 

 root-bundle in the same way as the four main pairs. 



2. The united portions of the bundles in the stem of certain Aroids and 

 Pahdanese (p. 268) are distinguished by a distribution and orientation of their 



