6o6' SECONDARY CHANGES. 



Sect. 194. On the other hand, the processes of growth and secondary formation 

 in the old internal parenchyma of the wood, at a distance from the active normal 

 ring of cambium, as seen in fleshy roots'", belong to this class. They have already 

 been described, p. 600, in Beta. Here they do not extend further than the growth 

 of the parenchyma itself, and the dislocations of other tissues which result from it. 

 New formations of secondary meristem and of cambium, and strands of wood and 

 bast occur, however, as first shown by Trecul = in the old root of Myrrhis odorata, 

 in the fleshy roots of Convolvulaceae, and species of Rumex, according to the 

 investigations of Schmitz \ and according to Stahl * in the roots of Bryonia, and 

 may be found more frequently in similar parts. Also the partial rings described 

 in Sedum Telephium should be connected with the above; the spotted structure 

 of thick Sumbul-roots may also find its explanation in the appearance of partial 

 strands of wood in the old originally normal wood. 



The roots oi Myrrhis odorata always have at first the normal structure and secondary 

 thickening, and they may retain this through life, and thus attain a great thickness. But, 

 in most cases, after the normal thickening has continued for a long time, the formation 

 by means of tangential divisions of radially seriate secondary meristem appears in the 

 internal parenchyma of the wood at some distance from the middle : it begins from one 

 point, and extends through an annular zone, which surrounds the root. This zone 

 assumes the properties of an independent normal cambium, which, starting from its outer 

 side bordering on the peripheral wood, and proceeding inwards, that is in centrifugal 

 succession, forms strands of bast of normal structure, alternating with medullary rays, 

 and subsequently on the side remote from the bast layer, that is facing outnuards, it forms 

 strands of wood, which insert themselves exactly on the surrounding portions of the 

 strands of wood, and increase in a centripetal, that is a reversed direction. This pheno- 

 menon is found in almost all roots i"™ or more in thickness. According to Tricul a second 

 internal layer of secondary meristem or cambium may appear in the same way as the 

 first": this also, if I understand rightly, forms a new layer of bast in centrifugal succes- 

 sion. Finally, a new cambium, which also forms a layer of bast, may appear between 

 the inverted strands of wood and those limiting them externally. 



If this has happened, the root then consists of the following concentric layers : 

 I. Normal cortex with a layer of bast. .s. Normal cambium. 3. A layer of wood, placed 

 normally. 4. A second internal cambium. 5. A layer of bast. 6. A layer of wood, 

 placed normally. 7. A layer of bast. 8. A third internal cambium. 9. A layer of 

 wood, inverted. 10. Cambium. 11. A layer of bast. 12. Axile strand of wood. This 

 discovery of Trecul's may be only a special case of the various possible combinations of 

 concentric zones of secondary formation. The zones that happen to be peripheral 

 follow the internal secondary formation by dilatation, while the normal increase continues 

 in the normal cambium. Further, any zone surrounding single strands of wood may 

 form a cambial zone from secondary meristem, by means of which the single partial 

 bundle undergoes an individual thickening, and becomes surrounded by bast. 



In the roots and also many stems of Con-vol-vutacess " investigated by Schmitz, various 

 anomalies appear ; viz. in the first place new strands of wood and bast, which grow 

 by partial cambiums, and appear first in the parenchyma of the old xylem ; secondly, 



' [Compare Weiss, Anat. u. Physiol fleischig verdickter Wurzeln. Flora, 1880.] 

 " Comptes rendus, 23 July and 6 Aug. 1866, torn. LXIII. 



= Sitzsber. d. naturf. Ges. 2u Halle, July 1874. Compare Botan. Zeitg. 1875, p. 677. 

 * By word of mouth. 



= [Compare Dutailly, Sur quelques phenomenes dans les tiges et les racines des Dicotyledones, 

 Paris, 1879.] 



