8o7 



the trunks which had been kept inclined intentionally the new structure 

 hangs free from the axis, like the drippings of a slanted burning candle and, 

 in response to the force of gravity, grows downward like an isolated pendent 

 braid, perpendicular to the earth's surface. 



In order to show that the various small spots, as has been observed by 

 Meyen, Th. Hartig and others, possibly are not merely productions of the 

 medullary rays, one such structure is shown in cross section in Fig. i88, aftd 

 in longitudinal section in Fig. 189. Fig. 188 H indicates the old wood, the 

 barked surface of which {t to t) is partially dead; only the middle portion 

 has started new production {N-N). 



The production began with a raising of the outermost cell layer by the 

 rapidly forming products of division of the immediately underlying sap- 

 wood layer and, in fact, also of the young wood cells together with the 

 vessels and the medullary ray cells. 



After the cork zone {k), which is becoming thicker, has surrounded 

 the comparatively scanty new parenchymatous tissue (r to p) , an inner 

 meristem zone appears very- early at first in bands and then connected. 

 This meristematic zone is new cambium {c to c), which now takes over the 

 secondary growth of cork. 



In this way the two processes of growth, which can take place in the 

 formation of bark on barked surfaces, differ considerably. If, as is the 

 case in enclosed wounds, which have been kept moist, the new bark begins 

 with a great production of callus, together with a long continued division 

 of the peripheral cells, as Fig. 186 shows, the formation of the outer cork 

 zone and especially the production of the inner meristem zone takes place 

 very late. In contrast to this, as in the present case, the wounded places 

 which have been exposed unprotected to the hot summer sun show the 

 second process, since the outermost remaining cells quickly thicken their 

 outer walls, collapse and in this way furnish for the immediately underlying 

 layers the necessary protection against drying. In this only a slight forma- 

 tion of parenchyma, but a very rapid appearance of the cambial zone, takes 

 place. It seems that the inner meristem zone has developed the more 

 quickly into a callus the more rapidly a sufficient bark pressure is produced 

 by suberization. 



The next production of the new cambial regions (Fig. 188, c-c) con- 

 sists in the formation of isolated new vascular bundle strands, which, 

 beginning with scattered short vessels {g), rapidly increase with age the 

 number and size of thgir elements and thus assume a wedge-like form which 

 constantly narrows toward the medullary ray regions (m) and at the begin- 

 ning is very broad, until structure and arrangement of the elements have 

 reached the normal stage of the unbarked trunk. To each xylem part 

 belongs a phloem part {ph), near which appear numerous cells containing 

 calcium oxalate (o). 



We see that the appearance of the vascular bundles in the parenchy- 

 matous ground tissue is the same as in the callus. This is true wherever 



