446 



BOTANY. 



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 

 phloem parenchyma, 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 

 layers of the cambium, and advances to- 

 ward the central layers. It never hap- 

 pens, hoAvever, that all the cambium lay- 

 ers pass over into permanent tissues,, there 



"~"\j -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 



f the formation of several transverse parti- 

 rig. 325. The row of . . . J 



ceils marked as at in tions in each cambium cell, and it is prob- 



Fig. 324 ; r, phloem : h, , , , . j- 



xylem ; at i are seen the able that in many cases there is a direct 



fissions of the cambium n , 



cells, x 600. After conversion oi cambium cells into sieve 

 tubes. That the cambium cells may be 

 converted directly into trache'ides is evident from Fig. 326, 

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

 fibrous tissue (If) and trache'ides (t) have the same origin, 

 and the indications are that even the large pitted vessels 

 (gg) are 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- 



