l68 PLANT ORGANS OR PARTS OF PLANTS. 



comes the irregularly formed outer part of the secondary 

 bark. The secondary bark is penetrated by medullary 

 rays, which are mostly one cell, seldom two cells broad, 

 and from one to fourteen cells long. 



In the keratenchymatous strings there usually grow 

 simple, tangential rows of oxalate cells, with small 

 tangential bands of parenchyma enclosing sieve bundles. 

 The similar tangential bands, in the entire bark, are made 

 up of concentric rows. On the tangential or radial 

 sections of the bark it may be seen that the thin-walled 

 oxalate cells are placed together in long rows, or, what 

 amounts to the same thing, they form long chambered or 

 divided crystal tubes. The zones of sieve tube con- 

 taining parenchyma are made up of rows, two to twelve 

 cells long, of starch-bearing cells, having thickened walls, 

 and of sieve bundles with rather broad, slightly bent, 

 separating walls, and sieve tubes with simple sieve plates. 



In old barks a change takes place. The single paren- 

 chyma cells of the phelloderm, and also those of the 

 bark fibres, increase in size, and their walls thicken, often 

 obliterating their lumen. At the same time, next the 

 sclerenchyma cells, single cells are produced containing 

 oxalate crystals. Later on a formation of cork takes 

 place, which, when the layer of cork becomes about ten 

 cells thick, is broken off. 



From the standpoint of the anatomist the root bark 

 varies, but slightly from the stem bark. It appears at 

 first as though the bast fibres in the root bark were 

 smaller than those of the stem bark, and that in old 

 barks the elements contained more numerous and 

 smaller stone cells. The very limited thickness of the 

 phelloderm layer is characteristic both of young and old 

 root barks; also the absence of the sclerenchyma cells, 

 lying outside of the cambium, and the absence of chlo- 

 rophyll in the phelloderm. In the root bark also the 

 medullary rays extend entirely to the cork layer. 



