8oi 



the harked wood stem and not merely that of the medullary rays in the 

 formation of callus. Nevertheless, under special conditions the medullary 

 ray cells can alone cause the formation of callus and yet the case often 

 occurs where the callus formation is initiated by the young wood cells alone. 



The young wood cells, the medullary ray cells and the narrow elements 

 participate in the callus formation by a transformation into parenchyma 

 cells which now increase in number \ 



The youngest cells, left on the wood cylinder, widen, elongate and 

 divide. The end cell of the last row of callus cells becomes largest. It is 

 often spherical, or club shaped, and the new cross wall is produced generally 

 in this stage. The new end cell now formed by the cross wall repeats the 

 process. The older cells, lying back of it, elongate and divide. 



Besides this kind of callus formation, Trecul observed still another 

 case. While the outermost remaining cells develop into callus tissue, by 

 distention and division, it also happens that they show only a slight develop- 

 ment, while the innermost young wood cells, lying beneath them, take over 

 the role of the actual callus former, Trecul sketches (pi. 7, Fig. 11) a 

 longitudinal section of the elm, the callus on the edge of which consists of 

 short, isodiametric cells. This gradually drying layer has been pushed up 

 from the wood by means of a thick callus layer, of which older cells now 

 adjoin the wood. The youngest cells most distant from the old wood, 

 lying directly under the outpushed dying layer, have stretched radially and 

 formed radially parallel rows. 



Both cases of callus formation can occur at the same time in the same 

 specimen. Probably the innermost layers of the exposed cambial body are 

 incited to increase by the drying of the outermost layers. 



As my experiments show, all the cells of the cambial region can partici- 

 pate in the callus formation, not only the young wood cells, as de Vries 

 thinks, but also the young bark cells. It depends alone upon which cell 

 layers are left when the bark is removed. If it is loosened in such a way 

 that only a few of this year's sapwood cells still capable of increase remain 

 on the old wood, the callus must be formed from them; if, on the other 

 hand, the very youngest cambium cells remain in place, they take over this 

 formation of callus, while the underlying young sapwood develops, accord- 

 ing to its position, into differentiated wood with vessels and is changed only 

 in so far as all its elements become shorter, broader in the radial dimension 

 and thinner walled. 



Trecul-, in his Fig. 5, pi. 3, of a linden, gives the best example of this 

 case. We will use this (see Fig. 186) to confirm our theory. B indicates 

 the young wood of the current year formed before the removal of the bark, 



1 "Les fibres ligneuses, les rayons medullaires et les vaisseaux d'un petit 

 diametre eux-memes sont metamorphoses en tissu cellulaire proprement dit; car il 

 y a une metamorphose reelle de ces organes elementaires en tissu utriculaire 

 ordinaire, et ensuite multiplication de ces utricules nouvelles. 



2 Trecul loc. cit., p. 167. 



