652 



SECONDARY C4ROWTH IN THICKNESS 



Attention may next be directed to the detailed structure of the 

 cambium, and to the characteristics of its component cells. Our 

 knowledge of the shape of cambial elements is mainly derived from 

 Velten's detailed investigations. The form of a typical cambial 

 cell is approximately that of a right prism, with its broader faces 



P 



^04333^ 



9" .a 



Pig. 270. 



A. T.S. across the central cylinder of a primary root of Vicia Fdba, after the com- 

 mencement of secondary thickening (diagrammatic) ; g, hadrome-plates ; b, fibrous 

 .strands, situated just outside the leptome-strands ; v, cambium; p, pericycle; 



s, endodermis. JS. Sector of A (x) more highly magnified ; lettering as in A. 



placed tangentially. The ends of the cells are gable-shaped, the 

 terminal walls following an oblique tangential course. A tangential 

 longitudinal section of the cambium, therefore, at once shows the 

 prosenchymatous character of the cells, whereas in radial longitudinal 

 sections the end walls are seen cut across, so that their sloping 

 character is not apparent. While the tangential walls are thin and 

 delicate, the radial walls are always according to De Bary, Ivriiger 

 and others more or less thickened, and furnished with longitudinal 



The thickening of the radial walls is more 



rows of circular pits. 



