446 



BOTANY. 



lar and interfascicular cambium layers are composed oi 

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

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

 334 and 335). In a tangential section the cambium cells 

 present an elongated outline, and their extremities are 

 usually more or less oblique (Fig. 336). From these cells 

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

 phloem iiarencliyma, sieve and fibrous tissues may be pro- 

 duced by more or less gi-eat modifications (Fig. 337). On 

 the other side (the xylem side) new ves- 

 sels, fibres, and parenchyma are also devel- 

 oped (Fig. 338). 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, however, that all the cambium lay- 

 ers pass over into peiananent tissues, there 

 always remaining one or a few moristem 

 layers. 



546.— A study of Figs. 336-338 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 337 that 

 the phloem parenchyma is jiroduced by 



„ the formation of several transverse parti- 



Fig. 3S5.-The row of . J -J. • , 



cells marked x—x in tions m each camDium cell, and it is prob- 



Flg. 3*1 ; r. phloem '.h, ,,,-., . • xi • j • j. 



xyTem;at« are seen the able that in many cases there IS a direct 



fisHJons of the cambium - j> i • n ' j. ■ 



cells, xeoo. — After conversion oi cambium cells mto sieve 

 ■"'^^'l'^" tubes. That the cambium cells may be 



converted directly into tracheides is evident from Fig. 336, 

 and also Fig. 75 (p. 84). In Fig. 338 it is plain that the 

 fibrous tissue {If) and trachGides {f) have the same origin, 

 and the indications are that oven 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- 



