446 



BOTANT. 



^ 



SJ 



lar and interfascicular cambium layers are composed of. 

 elongated cells, which multiply by fission in a tangential di- 

 rection, and thus give rise to radiating rows of cells (Figs. 

 324 and 325). In a tangential section the cambium cells 

 present an elongated outline, and their extremities are 

 usually more or less oblique (Fig. 326). From these cells 

 there develop various tissues. Thus, on the one side, the 

 pliloom fiarenchyma, sieve and fibrous tissues may be pro- 

 duced by more or less great modifications (Fig. 327). On 

 the other side (the xylem side) new ves- 

 sels, fibres, and parenchyma are also devel- 

 oped (Fig. 328). The development of 

 )~=^_,, these tissues begins in the inner and outer 



^(____-3j=:^ layers of the cambium, and advances to- 



ward the central layers. It never hap- 

 pens, however, that all the cambium lay- 

 iv__==>x ers pass over into permanent tissues, there 



1^ If always remaining one or a few meristem 



■' ' - ^ layers. 



546.— A study of Figs. 326-328 will 

 show the probable mode of development of 

 the permanent tissues from the meristem 

 tissue of the cambium. It is evident from 

 a comparison of Figs. 326 and 327 that 

 the phloem parenchyma is produced by 

 the formation of several transverse parti- 

 tions in each cambium cell, and it is prob- 

 able that in many cases there is a direct 

 conversion of cambium cells into sieve 

 tubes. That the cambium cells may be 

 converted directly into tracheides is evident from Fig. 326, 

 and also Fig. 75 (p. 84). In Fig. 328 it is plain that the 

 fibrous tissue {If) and tracheides if) have the same origin, 

 and the indications are that even the large pitted vessels 

 (gg) axe formed from cambium cells by the great increase 

 in the diameter of the latter, the thickening of their vertical 

 walls, and the partial or complete absorption of their trans- 

 verse walls. The origin of the xylem parenchyma from cam- 





Pig. 335.— The row of 

 cells marked x — a in 

 Fig. 324 ; r. phloSm : h, 

 xylem ; ati are seen the 

 fissions of the cambium 

 cells. X 600. — After 

 De Bary. 



