4^2 



SECONDARr CHANGES. 



into elements of the tissue. But even here, in the case of Pinus sylvestris, deviations 

 from the latter rule may be demonstrated. Of the first products of division of the 

 tissue mother-cell, one divides once more before passing over into definitive elements 

 of the tissue : in the case of the wood this is always the outer one ; in the case of 

 the bast it is usually the inner, more rarely the outer ; or each of them divides once 

 more. Thus in the former case three, in the latter case four elements of the tissue 

 are derived from one tissue mother-cell. 



The investigation of good transverse sections through the active and the resting 

 cambial zone of the most various Dicotyledonous and 

 Coniferous woods confirms Sanio's main result, ob- 

 tained in the case of Pinus sylvestris, as regards the 

 bundles'ofthe wood (comp. Figs. 196, 197). In every 

 radial row there is one initial cell, dividing tangentially, 

 from the division of which a new initial cell and a 

 tissue mother-cell proceed in each case. After one 

 or two further divisions of the latter, the definitive 

 elements of the tissue are formed. From what is 

 known of Pinus sylvestris, very various differences of 

 detail as regards the divisions by which the latter 

 elements are produced are to be expected as soon 

 as more extended minute investigations of this diffi- 

 cult subject have been undertaken. 



In most of the cases investigated the longitudinal 

 divisions in the tissue mother-cells take place ex- 

 clusively in the tangential direction, the elements of 

 the tissue are therefore always originally arranged in 

 radial rows, and deviations from this rule are the result 

 of subsequent displacement. Exceptions, however, occur 

 in the bast of many plants, on the origin of the sieve- 

 tubes ; here the mother-cell of a member of the latter 

 is divided by one or more excentric walls, directed 

 neither radially nor tangentially, into a member of a 

 sieve-tube, and cambiform cells. Comp. p. 324. This 

 no doubt applies to all the numerous cases of irregular 

 grouping of the sieve-tubes ; it is, however, undecided 

 how far displacements occur here, in consequence of 

 subsequent longitudinal growth of the elements, which 

 might give rise to the same result in the arrangement 

 of the latter. 



The variations mentioned in the diflferentiation 



and division of the tissue mother-cells are sufficient to 



show that the initial cells of a cambial ring do npt 



always fit exactly one on another with their radial 



surfaces, and that even the equivalent products of their divisions in the entire 



series of radial rows form with one another annular zones which are not smooth, 



but are often interrupted. Comp. Fig. 196, and Sanio, I.e., Taf. 5-8. In addition 



Fig. 195. — Pinus sylvestris; cambial 

 zone ; cros^-section through a radial row ; 

 after sanio (650). //side towards the wood, 

 i (conjectural) cambial initial cell. On the 

 side of i towards H are twin-cells of the 

 wood ; on the side of ?' towards the bast are 

 twin-cells of the bast; the cell bordering 

 on i towards the bast is the still undivided 

 tissue mother-cell for the bast, if the inter- 

 pretation of i stated above is correct; 

 otherwise the former is the initial, and i 

 an as yet undivided tissue-mother-cell for 

 the wood. 



