SECONDARY THICKENING. NORMAL DICOTYLEDONS. 473 



explain the extent of the angle of inclination ; the similar direction of the inclination 

 in the individual layers, its reversal, and more especially its alleged diminution in 

 the later layers in Pinus, &c., all remain to be explained. The basis of such an 

 explanation is to be sought in a more complete determination of the size and form 

 of the organs in question, than has yet been made. 



The undulated course of the woody fibres, which appears on cicatrised wounds, 

 &c., and gives the character to knotted and gnarled wood, may here be excluded 

 from minute consideration, as it is a pathological phenomenon ^. 



Sect. 139. Plants with typical Dicotyledonous structure of the stem, as well as 

 the majority of those Dicotyledons and Gymnosperms which are anomalous in this 

 respect, form, with very rare exceptions, a cambial ring in /he root- at an early 

 period, and this, when once present, shows a completely similar growth and new 

 production to that in the stem, although in each particular case definite special 

 differences exist between stem and root, which are to be discussed below. 



The first origin and orientation of the cambial ring must, on the other hand, be 

 different from that in the stem, in consequence of the different original general 

 structure in the root. It originates in the axial vascular bundle. The process 

 begins with growth in thickness and tangential division of that layer of cells which 

 borders on the inner surface of the phloem-groups, and in fact it proceeds from the 

 middle of each of these groups towards its two lateral margins, and thus also towards 

 the outer corners of the xylem-plates. The products of the tangential division are 

 cambium and young secondary thickening. In the usual case of abundant growth in 

 thickness, the tangential divisions eventually reach the pericambial cells lying outside 

 the xylem-plates, and are continued over the latter, thus uniting the originally separate 

 portions of the cambium to form a closed ring. As follows from the original 

 arrangement and form of the xylem- and phloem-groups in the root-bundle, the 

 general transverse section of the cambial ring, at its origin and before its completion, 

 is a figure following the outline of the xylem body; in the diarch bundles it is a 

 narrow ellipse, in those with more than two rays it is a polygon, with as many blunt 

 corners and concave sides as xylem-plates are present. As, however, the cambio- 

 genetic production of tissue on the side of the wood takes place unequally all round, 

 and is the more abundant the nearer it is to the original starting-points of the 

 formation of cambium, the phloem-groups are rapidly shifted towards the outside, 

 and the concavities of the ring become flattened, so that its transverse section soon 

 assumes a permanent, approximately circular form. 



It is only in the comparatively rare cases of slight growth in thickness that the 

 concavities between the original xylem-plates are permanent, and that the union of 

 the original portions of the ring round their angles fails to take place ; the formation 

 of cambium is in some few cases entirely wanting, comp. p. 355. In other cases the 

 tangential division in the neighbourhood of the xylem-plates is conspicuously less 

 than that opposite the original phloem-groups; the cells in the former position 



' Compare Schacbt, Lehrbuch, p. 67. — Goppert, Nachtrage zu d. Schrift iiber Inschriften, &c., 

 u. iiber Maserbildung, Bresl. 1870. — Idem, iiber die Folgen ausserer Verletzungen der Baume, Bresl. 

 1873. — Ratzeburg, Waldverderbniss, I. — Nordlinger, Forstbotanik, I. p. 274. 



^ See Van Tieghem, Ann. Sci. Nat. 5 ser. torn. XIII. p. 185, pi. 3, 4, 8. 



