108 PRACTICAL BOTANY. 



is a medullary ray. Note in row (3) the single initial cell (i), 

 oblong in transverse section, and the shorter diameter placed 

 radially ; according to Sanio's law of cambial division there is 

 only one such initial cell in each radial row : from this successive 

 segments (w w) which go to form wood have been cut off on the 

 inner side, others (b, b, b] which go to form bast on the outer side : 

 each is represented in the diagram as dividing into two by a 

 periclinal wall ; this is typically the case in Pinus, but the divi- 

 sion is not so regular in Dicotyledons. In row (1) is represented 

 a segment (w') 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 (6'), which, after 

 division, will go to form bast. 



Fig. 9, 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 : 

 i is, as before, the initial cell of the row : w w, pairs of cells 

 formative of wood : &, 6, 6, pairs of cells formative of bast : m. r. is 

 a medullary ray put in so as to show the relative position and 

 form of the cells. 



Fig. 9, C, represents the appearance of the cambium cells, i, 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. 9, 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 disten- 

 sion, and conversion of the 

 oblique walls into sieve-plates. 

 b. Parenchyma, by division of the 

 cells by transverse septa. 



