ANOMALOUS THICKENING IN DICOTYLEDONS AND GVMNOSPERMS. 585 



surrounding the cortex, they are squeezed into the very lacunar outer cortex of the 

 stem. The external much dilated bast portion, which is sharply marked off from 

 the less dilated younger bast, is Woronin's outer cambial zone. The general cortex, 

 ■which surrounds the bundle, follows by dilatation the growth of the latter, and of the 

 main ring of wood. In comparison with the latter, the growth in thickness of the 

 cortical bundles is slight : in the stem twenty-three years old investigated by Woronin, 

 the whole, transverse section of the strongest scarcely reached 2™"!, while the main 

 ring of wood was 45-55™™ thick. On the outer surface of the stem the cortical 

 bundles form blunt weak prominences. In the very old stem investigated by Mirbel, 

 which was 3™^', that is about 80™™ thick, the four cortical bundles were as thick 

 as the Uttle finger. 



An indication of cambiogenetic growth in thickness is found also in the con- 

 centric cortical bundles of many MelastomacecB (comp. p. 339), since a zone of 

 cambium surrounding the xylem increases the number of the elements in a radial 

 direction. Considerable increase does not take place, at least not in investigated 

 species, since the bundles are thrown off at an early stage, together with the outer 

 cortex, as bark. 



Sect. 190. Partial cambiums with inverted orientation of their products 

 of increase are known to us through the writings of Schmitz ^ in the case of the 

 rhizome of species of Rheum. 



In the tuberous branches of the rhizome of R. officinale the collateral leaf-trace 

 bundles form a normal bundle-ring, further increased by a normal cambium : it 

 encloses a large pith, which increases in relative circumference with the strength of 

 the branch. Through the pith when young there run transverse bundles, in closely 

 superposed transverse zones, corresponding to the nodes : these bundles connect the 

 bundles of the leaf-trace one with another, and with those of other traces in a 

 reticulate manner, partly by transverse branches, partly by vertical ones, which run 

 especially in the neighbourhood of the woody ring. Each of these connecting 

 bundles is originally a bundle of elongated cambiform cells and sieve-tubes ; they 

 are continuous with the phloems of the bundles of the leaf-trace at the point where 

 these curve out into the leaf At a very early stage a cambial layer appears round 

 each such strand of phloem, and forms on the one side strands of wood with much 

 parenchyma, on the other side strands of phloem corresponding to these, both 

 strands being split up by medullary rays. Further, the formation of bast is next the 

 original strand of phloem, the bast-layer is in fact on the inner side of the cambium, 

 but the xylem on the outer side. This extensive and continued growth produces a 

 bundle, which grows to a thickness of more than icm, and always retains the 

 characteristic inverted arrangement of wood and bast. The numerous medullary rays 

 with the same colouring matter as in the root (pp. 518, 524) give to the transverse 

 section a marked radial striation. Sections of this sort constitute the ' streaking ' and 

 radiate circles'' characteristic of the pieces of rhizome of the officinal Rhubarbs. 

 The growth in thickness of the streaky bundles continues even after the growth of 

 the pith has ended : the result of this is a partial compression of the pith and dis- 



' Sitzungsber. der natuif. Gesellschaft zu Halle, Dec. 1874. Botan. Zeitung. 1875, p. 260. 

 ^ Compare Wigand, Pharmacognosie. — Berg, Atlas, Taf. XII. 



