ENDS AND CONNECTIONS OF THE BUNDLES. 387 



elements, with reference to which van Tieghem' has appropriately termed them 

 compound bundles. The bundles, which on their downward course from the leaves 

 are at first collateral with normal orientation, usually unite to form a body of round or 

 irregular cross-section, their phloem-groups uniting either directly or by means of an 

 intermediate bundle of sclerenchyma to form a joint strand, at the periphery of which 

 lie the xylem-groups, which towards different sides are sometimes separate and 

 sometimes fused. The number and structure of the latter and the configuration of 

 the whole bundle change, both in successive cross-sections of the same individual 

 and in the different species. For details, van Tieghem, I.e., is to be consulted. 



Similar, less conspicuous phenomena no doubt occur here and there in other 

 stems of Monocotyledons, and in stems and petioles with medullary bundles. 



II. The following rules, which are to be stated with reference to Sect. 108, and 

 to the, developmental data given below in Sect. 117, hold good for monopodially 

 branched roois with regard to the attachment of the vascular bundles of lateral 

 branches to that of the main axis. 



Apart from a few exceptions to be mentioned below, every lateral root arises on 

 the outside of the vascular bundle of its relative main root, in front of the centre of 

 the outer edge of a xylem-plate. Accordingly, in the case of diarch and many 

 polyarch roots, their xylem-plates are attached to the corresponding outer edge of 

 the bundle of the main root, their phloem-strands to those alternating with it. 



In the polyarch bundles of many Monocotyledons, the xylem-plates are attached 

 not only to the corresponding plates of the main root, but also to the two neigh- 

 bouring lateral ones, while the phloem-bands are attached to those bands in the 

 main root which alternate with the three xylem-plates mentioned ; van Tieghem 

 found this to be the case, for example, in the ramifications of the adventitious roots 

 of Iris Germanica, Asphodelus ramosus, and Asparagus, The division of the lateral 

 bundles, in the case of Pandanus and the Palms, into branches which penetrate 

 further and deeper, was mentioned above at p. 316. In the Grasses, on anatomical 

 grounds, which have still to be explained below, Sect. 117, the lateral roots do not 

 stand in front of the xylem-plates, but in front of the centre of the phloem-strands of 

 the bundle of the main root ; accordingly their xylem-plates are attached to the two 

 next lateral plates of the main-root, their phloem-groups to the corresponding ones of 

 the latter. A similar relation obtains, for similar reasons, among the Pittosporeae. As 

 will also be further explained in Sect. 117, each lateral root in the Umbelliferse and 

 Araliacese is placed between a xylem and phloem-group of the main root ; its xylem 

 is accordingly obliquely attached to that surface of the corresponding plate of the 

 main root which faces it ^. The same position of the lateral roots and insertion of 

 their bundles occurs, according to van Tieghem', in Lycopersicum, while nearly 

 related plants, e. g. Solanum tuberosum, show the usual behaviour of oligarch roots 

 mentioned above. 



In the case of the other connections of bundles between main and lateral axes, 

 "nothing essential need be added to what has been said in earlier paragraphs, and at 

 the beginning of this, respecting the orientation of the parts of the bundle. 



' Structure des Aroidees, /. c. ' Van Tieghem, Symmetrie de Structure, /. c. 



^ I.e. 226; cf. also § 117. 



C C 2 



