STRUCTURE OF CONCENTRIC BUNDLES. 339 



plant. No accurate investigations of them however lie before us, and the example 

 of Welwitschia (p. 335) shows that one must be cautious in coming to a decision on 

 apparently bicollateral bundles. On the behaviour of Trapa, which here remains to 

 be mentioned, comp. sect. 105. 



2. Concentric Bundles. 



Sect. 104. In concentric bundles one of the two parts occupies the middle, and 

 is encircled by the other. 



Of the two cases here possible, the one, namely that in which the phloem occu- 

 pies the middle and is surrounded by the xylem, occurs in the lower ends of the leaf- 

 trace bundles of many, but not of all rhizomes of Monocotyledons, where they lie at 

 the periphery of the bundle-cylinder in the stem, e. g. Iris germanica, Cyperus aureus, 

 Papyrus', Carex arenaria''' (but not, for example, C. disticha and C. hirta), Acoras 

 Calamus and A. gramineus '. This form arises no doubt from the collateral bundle, 

 as in its course the xylem gradually surrounds the phloem more and more on both 

 sides, until the latter is completely enclosed ; where it does occur however this form 

 must be distinguished from the typically collateral. The structure and the surround- 

 ing tissues present no generally valid difierences from collateral bundles. The phloem, 

 which is round as seen in cross-section, is as a rule surrounded by a single, or 

 rarely by a multiple ring of reticulated or pitted vessels, with parenchymatous 

 cells interspersed between them. Comp. fig. 148, p. 317. 



Sect. 105. The other possible case, that the xylem occupies the middle and 

 is surrounded on its whole surface by the phloem, occurs in individual Dicotyledons 

 with anomalous distribution of the bundles, also in isolated cases among the Cycadese, 

 and is characteristic of the entire group of Ferns, with a few exceptions, partly 

 mentioned above. 



Among Dicotyledons the medullary and cortical bundles of the Melastomaceae * 

 may first be mentioned. In these the centre is occupied by a few narrow vessels, 

 which are scattered among delicate prismatic cells, the vascular group being sur- 

 rounded by a delicate ring, consisting of sieve-tubes and cambiform cells. In feeble 

 bundles only one single, narrow, spiral vessel often occurs, and even this may be 

 absent, so that we then have the sieve-tube bundles mentioned at p. 231. 



According to Reinke's description, all the bundles of the stem of the species of 

 Gunnera, especially G. scabra, also belong to this series ; so also do those in the stem 

 of Auriculas (cf. p. 251). In the leaf of the plants last-named the bundles are collateral, 

 and are arranged in a row as usual among Dicotyledons. The collateral structure 

 also holds good for the smaller bundles of the stem, even when almost circular in 

 cross-section ; on the one side is a small group of narrow, primitive, spiral tracheae, 

 with larger reticulated vessels external to them; on the other side is the small 

 phloem, the whole being surrounded by delicate cells, bounded externally by the 

 endodermis. On the other hand, the larger bundles of the stem of Pr. auricula 

 show a concentric arrangement; the narrow primitive elements are in the middle, 



' Link, Icones AnatomicsE, Tab. V. figs. 1,9; IX. fig. 6. 



* Treviranus, Physiol. I. p. 195, Taf. III. ' Van Tieghem, /. f. 



' Sanio, Botan. Zeitg, 1865, p. 1 79.— Vochting, Melastomeen, /. c. 



Z 2 



