145 



dirootionJ)nt also in the direction of the long axis, it is 

 evident that the radial cell rows curve out to^wd the cut 



I^I?riv?"*^^.*5^^ l^?^l^-2' ^^^^ aP^-^rt. as at z in the figure. 

 Bejrond this point of tearing, the cell divisions are very ir- 

 regular*. -In the callus of the elm hore shown, however, the 

 cell rows may he clearly recognized although thev are turned 

 toward the outsidd, 



The further increase of the callus tissue is often brought 



J oy th^ mrnftnTrnort n-P r\ -nan vn<-j-^-4 e..^.^™^..*-^ « ■ -^^-^ *.i-.-. ».->«4 



.- Of wh 

 indicated. 



The further increase of the callus tissue is often brougl 

 ahout by th^ appearance of a nev meristematic zone, the posi- 

 tion of which may vary. In figure 64, at a-a, such an one is 

 indicated. 



In- addition it should he emphasized that the youngest 

 barK layers act just as do the cambial colls, - from which they, 

 may not always be sharply distinguished,- flirnishing the same 

 short-celled parenchyma as do the cambial elements. The coni- 

 fers, .so far as investigated, assume an unique position, in 

 that the latest, unlignifiod elements of the wood-body are also 



incited to an analogous change by wound-stimuli. They divide 

 and furnish parenchyma cells^. 



Figure 65. a tangential section, mado above a girdling 

 wound in Abies CQphalonioa . shows in itv<? upper part normal trach 



(161) eide with borderoa pits, in its lower, parenchymatic cells, 

 produced by segmentatioij' of the young woody elements which tove 

 unbordered pits. , The transition between the tv/o zones furnish- 

 es some cells which show the charaxster of tracheids in their 

 upx5©;r parts, but in their lower ones, the pitting of parenchyma 

 cells. 



Wot only the youngest elements of the se condary bark . of 

 which we have just spoken, . participate in the formation of cal- 

 lus, but at times all of its layers or at least the growths of 

 the last few .years. Of all the cuttings which I have investi- 

 gated, those of Populus form the most abundant bark callus. 

 Here, as from the cambium., regularly radial rows are also pro- 

 duced by tangential c:q11 division, which curve out on the cut 

 surface like rojbls, as was stated above for cambial callus. 

 (Compare fig.. 66), The Tjark callus usually merges with the 

 adjacent carabiarl callus into an homogeneo^^s tissue roll, so that 



(162) it is possible to determine only by microscopic investigation • 

 which part of a callus pertains to the bark or to the cambium 

 and whether the former was at all active^. 



In plants v;ith fixed medullary phloem, its cells may be 

 incited by injury to division and the formation of callus, 

 (Observations on Eucalyptus ) , 



The primary bark , especially in Salix, passes over easily 

 to the formation of callus. Only a few days after the injury, 



^ The changed tensile and pressure conditions might not 

 be without influence on the direction of the cell division. 



^ Mfiule, Der Paserverlauf im Wundholz. Bibl. Botan, , 1895, 

 Heft. XXXIII. 



^ Also in Populus, the participation of the bark in the 

 formation of callus varies greatly in different e^ra-mples; often 

 it is entirely li^cking. Xn <^Q case, I saw the cambial callus 

 only weakly developed, and the bark callus therefore excessive- 

 ly richly increased. 



