ANOMALOUS THICKENING IN DICOTYLEDONS AND GVMNOSPERMS. 575 



the middle and the lateral lamellae broad and obvious. Pith broad, 

 breaking down in the middle: Anemopivgma Mart. 



4. Stem not rope-like. Numerous bast-plates with very narrow steps and a 



broad medullary ray on each side of their middle lamella : Limdia DC. 



5. Stem rope-like. Wood with wide vessels. Bast-plates variably con- 



stricted, and destroyed by pressure by the adjoining steps of wood 

 (sometimes formation of wood in the cortex, opposite the bast-plates*) : 

 Distictis Mart. 



(Z>) Young shoots with six ridges thrown off later: old ones cylindrical, cortex 

 thin. Numerous very unequal bast-plates with irregular steps : Amphl- 

 lophium Kth. 

 (b)* Stem with concentric (renewed) rings of wood in the cortex. 



1. Stem not rope-like or slightly so. Medullary rays of almost equal breadth : 



Haphlophium Cham. 



2. Stem rope-like. Medullary rays of very unequal width : Gla-ziovia Bur. 



B. * Inner layers of wood split up in the old stem by subsequent intercalary forma- 

 tions of parenchyma, wood, and bast. 

 {a) Wood with annual rings. Segments of splitting few. In the bast-layer are 

 wood bundles, at first fan-shaped in transverse section, but soon rounded 

 off: Anisostichus Bur. (Figs. 224 and 237.) 



(b) No annual rings. Segments of splitting numerous, repeatedly cleft, between 

 them newly formed parenchyma. The segments at the middle of the stem 

 turned in all directions. 



1. Segments of wood at the middle of the stem three-cornered, undivided, all 



others dichotomously split: Melloa Bur. (Fig. 226.) 



2. All wood-segments, both those in the middle and those at the periphery of 



the stem, dichotomously split : Bignonia Bur. 



Sect. 184. The structure and growth of the internodes of species oi Phyiocrene'^ 

 correspond in their main features with those of the climbing Bignonias, The differ- 

 ences between the two consist partly in differences of the more minute structure of 

 wood and cortex, partly in this, that the number of the original protrusions of wood 

 and intruding plates of bast is not four, but usually eight in the specimens examined, 

 more rarely thirteen, numbers which appear to vary with the individual. Subsequent 

 divisions of the original protrusions of wood by secondary plates of bast have not been 

 observed, but on the other hand there are sometimes pauses in the activity of the first 

 cambium, and zones of renewed growth then appear in the cortex. Comp. Sect. 191. 



As regards the relations of structure the following facts may be brought forward, 

 reference being made to the description of Mettenius, and Mohl's supplement to it. 

 (Fig. 227). 



The pith is surrounded by a narrow uniformly thick, ring-like zone of wood (me- 

 dullary sheath), which includes between' the slightly thickened cells the primary vessels 

 arranged in radial rows, usually in pairs opposite the protrusions of wood. This zone 

 passes over externally into one which is also annular, and is composed of numerous 

 successively wider pitted vessels, thick-walled fibrous elements, fascicular parenchyma, 

 and narrow medullary rays. This zone of pitted vessels, retaining fundamentally the 



' Griffith, -in Wallich, plant. Asiat. rarior. III. p. 216, after Lindley, Introduction to Botany, 

 p. 69. — A. de Jussieu, I.e. — Treviranns, in Botan. Zeitg. 1847, p. 400. — Mettenius, Beitr. z. Botan. 

 p. 50. — v. Mohl, Botan. Zeitg. 1855, p. 878. 



