Page:The Oak.djvu/85

 cambium. The radial rows of the latter can be followed for some distance, the radial diameter of the cells increasing, 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 effective in causing displacements. The bast cells developed earlier, and therefore more distant from the cambium zone, now lie in the perceptibly large periphery, and thus undergo tangential extension or radial compression, and so undergo changes of form. Besides these alterations 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 farther out, where great irregularity 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 the above cells are found to be septate and cut up into parenchyma-like cells—irregular bast-parenchyma. The