ANOMALOUS THICKENING IN DICOTYLEDONS AND GFMNOSPERMS. 587 



to the special cases to be described. The successive rings may be briefly termed 

 concentric, inasmuch as the separate portions of all have a similar orientation 

 relatively to the organic central point of the whole transverse section of stem or root. 

 This term is not always strictly accurate. There is often strongly eccentric develop- 

 ment, or stronger development sometimes on one side, sometimes on another. Not 

 unfrequently instead of a complete ring only portions of one are formed, and these 

 are then often so far eccentric that their margins abut on rings or portions of rings 

 next within them. In flat or winged stems of Lianes, e. g. many Leguminosge ^, one 

 can in many cases hardly speak of portions of rings at the corners, but rather of 

 parallel striae or bands. 



Since the xylem-portions of the rings usually form the chief mass, as in normal 

 wood, and the bast-portions are relatively small, it follows that, in portions of rings 

 which are very closely packed and short, facing alternately to diff"erent sides, and with 

 their margins always fitting closely on the next inner ones, the whole of the xylem 

 portions in the transverse section of stem or root form a connected mass of wood, 

 in which the layers of bast belonging to them, together with the narrow intermediate 

 zones, run as curved, irregularly concentric, and relatively narrow bands. This is very 

 striking, e. g. in the Securidaca figured by Miiller (/. c. Fig. 6), and the Hippocratea- 

 ceous plant Tontelea, represented I.e. Fig. 7. Not only in the last-mentioned cases, 

 where the successive portions of rings are in immediate contact at their margins, but 

 also in the cases of more regular concentric arrangement, the radially successive 

 similar zones are in direct continuity, either at the nodes alone, or at numerous points 

 in their longitudinal course. 



According to the structure of the individual successive rings, which varies like 

 that of the normal wood, according also to their relative thickness, and that of the 

 intervening zones, according to the special structure of the latter, and the diff"erent 

 degrees of concentric arrangement, of length, orientation, curvature, and marginal 

 connection of successive rings and portions of rings, and finally, according to the 

 general form of the transverse section of stem or root, the structure of the parts in 

 question may differ with endless variety. It would lead us too far to enter into all 

 the individual cases which here present themselves. As opportunity off'ers therefore, 

 single examples may be brought forward, partly in this paragraph, partly in those 

 which treat of Chenopodiacese, Phytolacca, and Cycadese. Compare also the descrip- 

 tion of Fig. 227. 



As regards the place of origin of the successive renewed zones of growth two 

 different cases must be distinguished. 



(i) The more rare mode is this, that zones of growth following the normal one 

 arise in the primary outer cortex. It was discovered by Decaisne ^ in Cocculus 

 laurifolius, and exactly described later by Nageli^, Radlkofer*, and Eichler^ and occurs 

 generally in the Menisperme(B of this category ; further it is found in the Cycadece 

 to be described below, and in the peculiarly constructed stem of the Avicennias. 



' Compare Criiger, Botan. Zeitung. 1850, Taf. III. Figs. 19-21, Rhynchosia phaseoloides. 



* Mem. sur les Laidizabalees, Arch, du Museum d'Hist. Nat. i (1839). 



3 Beitr. I. I.e. * Flora, 1858, p. 139. 



^ Denksch. d. bot. Gcs. Regensbiirg, Bd. V. p. 1. 



