622 



contract to their former breadth only wliere the porous tissue passes over 

 into the secondary wood (Fig. 149 2, ?, A') with reg-ular ducts {g''). Then 

 a normal cambial zone is formed again (Fig. 149 2 c~) which, at the time 

 when the medullary rays were broadened excessively, had become irrecog- 

 nizable, since cell division took place absolutely irregularly in different 

 regions of the ring of thin-walled tissue. As soon as the formation of the 

 regular cambial zone begins again, die loose bark tissue also differentiates 

 itself in such a way that juvenile bast groups (Fig. 149, 7 h and 6 b h') 

 again becomes recognizable. 



The fact, that no dead tissue of any kind is present between the wood, 

 matured before the action of the frost (h), and the looser, thin-walled 

 tissue (Ig), proves that the young wood, the sap wood ring, has passed over 

 directly into the parenchyma wood of the ring of thin-walled tissue. Never- 

 theless, this parenchyma has retained its connection with the wood body. 

 On this account, it is not suq^rising that, after the cessation of the causes 

 which had brought about this parenchymatous formation of wood, the tissue 

 gradually re-assumes the normal wood character and adapts itself to the 

 formation of a secondary wood ring (Fig. 149 2 and ? h'). In fact, indi- 

 vidual elements of the sap wood, the thickening of which had advanced 

 somewhat further at the time when the formation of parenchyma wood 

 began, had continued the thickening of their walls. On this account, we 

 find isolated tracheal elements (Fig. 149 4, fr) in the centre of the paren- 

 chyma wood. 



The zone of thin-walled, porous tissue (Ig) in the cross-section of the 

 oak branch (Fig. 148) is, therefore, only a modified wood ring which has 

 passed over into an excessive new cell formation. Since such a cell increase 

 can arise only from elements which still possess their cambial nature, it 

 must necessarily be concluded that the very youngest cambial zone elements, 

 i. e., the sap wood, have produced this parenchyma wood. As a matter of 

 course the real anatomical cambium, together with the young bark, has par- 

 ticipated in this cell increase and, in this way, produced the abundant tissue 

 in which it is not possible to distinguish where the transition from wood to 

 bark takes place. 



We now ask what may be the cause of the formation of this profuse 

 tissue zone? The answer can only be found in the removal or weakening 

 of the constricting, compressing influence, exercised by the bark girdle, as 

 a whole, on the youngest tissue, i. c., the cambial region. 



This cause is indicated by the holes in the bark tissue (Fig. 148 /', at 

 the right). Such tangential holes in the healthy tissue are produced by the 

 upraising of the tissue lying above the hole from that lying beneath it. It 

 can only be raised, however, if it has not enough room on this underlying- 

 parenchyma which is caused by a greater tangential distention. Conse- 

 quently, a stronger tangential strain has occurred in these outermost tissue 

 layers than in the adjacent inner layers of the bark. 



