ELM STEM 65 



dividing into two by a periclinal wall ; this is typically the case 

 in Ptnus, but the division is not so regular in Dicotyledons 

 In row (i) is represented a segment (w 1 ), recently cut off from 

 the initial cell on the side next the wood, in which this division 

 has not yet taken place ; in row (4) there is a similar undivided 

 segment (b 1 ) which, after division will go to form bast. 



Fig. 7, B, shows diagrammatically the arrangement of the 

 cells of one of these rows (3), as seen in radial section : the 

 length of the cells is much greater than their width, and the 

 ends are square : / is, as before, the initial cell of the row : w TV, 

 pairs of cells formative of wood : b y b, b, pairs of cells forma- 

 tive of bast : m. r. is a medullary ray put in so as to show the 

 relative position and form of the cells. 



Fig. 7, C, represents the appearance of the cambium cells 

 i, i, /, in tangential section : they are obliquely pointed, and 

 their width corresponds to that shown in A : m. r.^ as 

 before, the medullary rays. 



Fig. 7, D, shows the form of a single isolated cambium-cell 

 as a solid body, drawn to the same scale as the other figures : 

 if such a cell be cut transversely, radially, or tangentially, it 

 would give the appearance presented by the initial cells (i) in 

 Figures A, B, and C. 



Taking cells of this form as a starting point, the several tissues 

 above described are derived from them in the following way : 



(i.) Phloem a. Sieve-tubes, by lateral distension, and 

 conversion of the oblique walls into 

 sieve-plates. 



b. Parenchyma, by division of the cells by 



transverse septa. 



c. Fibres (sclerenchyma), by elongation and 



interweaving of cells, the width of 

 the cells at the same time being 

 relatively reduced : the end of the 

 cells slide past one another as the 

 cells elongate. 



(ii.) Xylem a. Vessels, by lateral distension, and absorption 

 of cell-contents, and of the oblique 

 walls. 



F 



