THE ANGIOSPERMAE : STEMS 921 



XYLEM ^^J^\\ \ ^\ PHLOEM 



Fig. 904. — Primula auricula. Single meristele which is probably a 

 reduced amphiphloic bundle. (After Bouygues.) 



7. Inverted Bundles. In certain cases, of which Rumex crispiis and Rheum 

 officinale are examples, phloem strands appear on the medullar}^ side of the 

 primary bundles, and on the outer edge of each phloem strand arises a 

 secondary cambium, which cuts off xylem elements outwards, the inner bundles 

 thus appearing like primar\' bundles, but with inverse orientation of tissues 

 (Fig. 905). A second inverted ring may even be formed inside the first. 

 Such inverted orientation also occurs in some forms of anomalous secondary 

 thickening, especially in climbers. 



8. Repeated Concentric Rings of Bundles. Examples of this structure have 

 already been cited in the roots of Chenopodiaceae such as Beta, and in the 

 stem of Gnetum gnemon. It is not uncommon in the secondarilv thickened 

 stems of woody climbers, as we shall see in Volume IV, but it also occurs 

 in thickened stems where it is not associated with a special growth habit, 

 e.g.,Phvtolacca dioica. Here a secondar>^ growth zone of tangentially dividing 

 cells appears at the outer edge of the phloem, i.e., in the pericycle of the first, 

 normal ring of bundles. This growth zone expands outwards into a zone 

 of parenchyma, at the outer edge of which a true secondary cambium is 

 formed, from which a second ring of vascular bundles is differentiated. This 

 in its turn is replaced by a third zone of bundles formed in the same way, 

 and so on till six or seven concentric bundle rings have been formed. Cambial 

 growth in each ring ceases when the next ring has been formed. 



30 A 



