T11K SKF.IHJNG AND YOUNG PLANT 67 



chiefly of delicate, apparently irregular parenchyma 

 cells with cellulose walls ; this is easily traced to the 

 cambium. The radial rows of the latter can be followed 

 for some distance, the radial diameter of the cells in- 

 creasing, the walls thickening, and the rectangular shape 

 changing. Displacements from the radial arrangement 

 then occur. A few cells assume a nearly circular form 

 (i.e. in transverse section), and the larger ones are effec- 

 tive in causing displacements. The bast-cells developed 

 earlier, and therefore more distant from the cambium 

 zone, now lie in the perceptibly larger periphery, and 

 thus undergo tangential extension or radial compression, 

 and so undergo changes of form. Besides these altera- 

 tions in form and position, the more delicate bast elements 

 increase in numbers by the development of perpendicular 

 division walls ; this is quite clear in those parts nearest 

 the cambium, but further out, where great irregu- 

 larity occurs, it is impossible to say which cells have 

 arisen direct from the cambium and which by these 

 later divisions. Still, certain thin septa betray their 

 late origin. 



On tangential sections we see elongated, pointed, 

 interpectinating cells, with secondary rays of parenchyma 

 between, showing that these are formed and continued 

 by the cambium. Each pointed cell has proceeded from 

 a cambium cell, and indeed only differs in its thicker 

 walls and pits. These cells are still simple, or here and 

 there have a transverse septum obliquely across. If the 

 tangential section is in a slightly older portion, most of 



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