384 



the cork layer lie the collenchymatically thickened bark layers (c o) ; these 

 border on the parenchyma containing the chlorophyll (chl) which is seen 

 separated into zones by tangential calcium oxalate bands (o, o\ o'). The 

 normal bark of the healthy branches also not infrequently has tangential 

 cavities along these bands of crystals, produced by the tearing of the cells 

 which remain thin-walled and contain small deposits of calcium oxalate, so 

 that some of the crystals appear to be lying free near the edges of the 

 cavities. 



In the autumn of the first year, the phloem rays may be seen to extend 

 as far as the first oxalate band (o). Adjacant to these rays, as is usually 

 the case in our woody plants, the cambial zone (c) curves outward over 

 the wood, and then in again, like a bow. This shows that the medullary ray 



assists in the radial extension of the axis, 

 just as the pith cylinder itself causes the 

 longitudinal stretching. 



On an average the normal medullary 

 ray (ui) retains, inside the bark, the 

 number of cells last formed in the wood 

 and its extension in the bark then depends 

 only on the greater distension of the in- 

 dividual cells. Near the excrescence, 

 however, medullary rays are often found 

 of which the cells have increased in 

 number (;;?') but have kept essentially 

 their radial, normal elongation. An ex- 

 traordinary cell increase finally takes 

 place in the ray of the excrescence and the 

 cambial zone curves abruptly outward. 

 This is best seen in the comparatively few cases in which the medullary 

 rays begin with the formation luiilaterally of excrescent tissue, as shown in 

 Fig. 65. In this figure m indicates the cells of the medullary ray within the 

 wood ; c the cambial zone which at the right side rises towards the wood 

 (h) and sinks back at the left side over the wood; nr is the normal side of 

 the bark ray, which pushes against the thick-walled bark parenchyma (/>) 

 and, in caustic potash, is clearly differentiated from its surrounding tissue 

 by its yellower color. At are indicated the very thin-walled small cell 

 rows containing calcium oxalate; here, near the cambial zone, the walls of 

 these cells show a peculiar granular consistency as an indication of their 

 approaching decomposition. Such a granular, slimy decomposition of these 

 cell bands and the movement of the calcium deposits to the edges 

 of the cavities thus produced is also found in the normal bark. On 

 the excrescent side (wr) of the bark ray, of which, the cells turn a still 

 darker yellow after treatment with caustic potash, than do those of 

 the normal side, and not infrequently display a distinct knot-like swell- 

 ing of the walls, the cambial zone turns abruptly outward {c') and indi- 



Fig-. 65. Meilulhiry ray in the first 

 .stajies of the gnarl formation. 



