;86 



SECONDARF CHANGES. 



placement of the bundles. The growth in thickness of the bundles further extends 

 to their point of insertion on the bundle of the trace as it passes out into the leaf, 

 and continues at the latter point even though the growing woody ring may have 

 enclosed both portions. The streaky bundles therefore lie both in the pith and 

 in the parenchymatous wood ; they run, as the result of their original arrangement 

 and subsequent displacement, in very different directions, their stellate transverse 

 sections are therefore found both in sections cut transversely, and in other directions 

 through the rhizome. The quantity of the streaky bundles is on the average the 

 greater, the thicker the rhizome, Schmitz found the same phenomena in the 

 rhizome of Rheum Emodi, but not in that of other species cultivated in this country 

 (Germany). 



Sect. 191. Successively I'enewed thickening rings ^ are a rather wide-spread 

 phenomenon, which appears in very various special forms. The general course of the 



type of growth thus indicated is as follows. 

 Both in the stem and root the secondary 

 thickening begins in all respects normally, 

 and often proceeds for a long time thus 

 normally. Then there appears in the 

 parenchyma, outside the normal first cam- 

 bium, a secondary meristem, formed chiefly 

 by tangential divisions ; it usually starts 

 from single points, and then extends 

 laterally : in it corresponding strands of 

 wood and bast separated by medullary 

 rays are differentiated. These are them- 

 selves arranged like a normal Dicotyle- 

 donous ring of secondary thickening : 

 wood and bast are permanently separated 

 by a cambial zone, which has as regards 

 them a normal orientation : by means of 

 it they receive further thickening. Their 

 structure resembles that of the normal 

 secondary thickening in the same plant in 

 all important points. Thus spiral vessels 

 Almost simultaneously with the appearance 

 ring, the secondary thickening of the first 



In the same way the 



Fig. 233. — Gnetum scandcns. Piece of a transverse sec- 

 tion of a branch (8) ; ni pith ; i, 2, 3, successive rings of growth, 

 the 3rd at the right just beginning to develope, but already 

 strongly developed to the left ; r ring of stony sclerenchyma, 

 at the inside of the outer cortex, which is covered by fissured 

 cork. — Cortex, medullary rays, and intermediate zones are left 

 white, also the bast-strands, but the latter are limited by a 

 single line ; the strands of wood united with them are trans- 

 versely shaded, excepting the transverse sections of the large 

 pitted vessels. 



are absent here also from the w8bd. 



of this second cambium and second 



normal ring ceases, at least in carefully investigated cases 



second may cease to grow, and be replaced by a third like it, and the same process 



may repeat itself through unlimited series. 



The successive rings, consisting of wood, bast, and limited cambium (Fig. 233), 

 may, as above, be called shortly rings of growth. They are separated from one 

 another by zones of dissimilar tissue, the special quality of which may vary according 



' Compare the quoted works of Gaudichand, A. de Jussieii, Mettenius, Criiger, F. Miiller, 

 Bureau ; further those to be quoted in this paragraph, by Decaisne, Nageli, Kadlkofer, and Eichler, 

 and in |§ 192-195. 



