868 



stages of development. It is evident that those first formed also become 

 smaller at first after entering the axis.- m is the pith; w b, the pith bridge 

 of a central leaf trace, of which the secondary bundles are unequally devel- 

 oped; m St, medullary rays; hb phloem fibre groups, which compose the 

 central core of the wood cord formed in the bark. In Fig. 206, 4 rt is the 

 bark killed by pressure and pressed into the trunk by the xylem strand 

 formed in the axis of the branch. Fig. 206, 5 g^ indicates a xylem strand 

 with the beginnings of overgrowth; this is seen to be more strongly devel- 

 oped on the outer side. Fig. 206, 5 gr' is a xylem strand which has not closed 

 completely into a wood cylinder. Its formation took place as follows : cell 

 increase began on the outer side of the phloem fibre group in the phloem 

 sheath and led to the formation of vascular elements and wood cells. The 

 one-sided wood body thus produced is closed by the gradual union of the 

 two edges, turned to the centre and growing toward each other. Fig. 206, 

 5 c' is the cambial zone of a xylem strand already closed internally but still 

 pressed into a kidney shape at the place of union. Fig. 206,^ gives a part 

 of Fig. 206, J g' somewhat magnified. 



In Fig. 206,-? is seen the complete correspondence with the centre of 

 the gnarled tuber in the apple, hb is the phloem fibre group ; p, the wood 

 parenchyma ; g, the vessels ; x, short, cross-cut wood cells ; x', wood cells, 

 extending horizontally from the inner convexity of the wood cord at the 

 place where the two edges have united ; m represents the rows of medullary 

 rays spread out like grasping arms ; c, the cambial zone surrounding the 

 strand; r, the youngest bark parenchyma of the specialized zone of bark. 



The xylem strands (Fig. 206,5) ^^^> therefore, produced at the base of 

 the swelling by an unusually abundant nutrition of the phloem sheath; 

 theii- primordia lie at unequal heights. When enlarging, they compress at 

 first the bark tissue (Fig. 206,5) which separates them from each other and 

 finally also the tissue lying above them, which separates them from the axial 

 cylinder and is found later as a brown mass in the centre of the wood body 

 (Fig. 206,^ rt). With their entrance into the axial cylinder, the form of 

 the xylem strands in the bark is changed; the core becomes eccentric and 

 finally pressed back to the tip of the wedge-shaped strand as shown in Fig. 

 206,^ g', g-, g^. The change of form is, therefore, exactly the reverse of 

 that undergone by the normal vascular bundle which enters the bark from 

 the axial cylinder. 



Farther out the branch becomes normal^ . 



The occurrence of bark-produced wood strands, therefore, explains as 

 follows the production of the gnarl tuber. The mature tuber is a wood 

 sphere isolated in the bark, of which the upper surface is composed of a 

 cambial and bark mantle, receiving its nourishment from the surrounding 

 bark tissue. According to the investigations of the above-named scientists, 



1 On the similarity of this formation of the secondary wood with that in the 

 Papindaceae compare Sorauer, Die Knollenmaser der Kernobstbaume. Landwirtsch. 

 Versuchsstationen 1-878. 



