796 



Fig. 184 shows the callus in cross section at the place where it leaves 

 the old bark, i. e. about at 6* to S' in Fig. 183. Fig. 185 is a cross section 

 through the middle of the projecting part of the callus, i. e. about in the 

 place k to wh in Fig. 183. In Fig. 184, H represents a part of the old wood 

 formed before girdling, g' indicates the wide, scalariform vessels of which 

 those lying nearest the cut surface vS to S' have filled with tyloses (t) as a 

 result of the injury, and consequently have become impervious to air; h 

 shows the tracheids in cross section. S' to C (in Fig. 185, C to C) is the 

 new wood formation of the callus. We find that the medullary rays (w), 

 from the normal tissue (H), are continued, after a short interruption, into 

 the callus. The medullary rays become constantly broader; the vascular 

 bundles, the xylem elements of which in normal wood are closely packed, 

 are separated further and further by the constantly widening medullary 

 rays. The bundles thus have fewer elements and normal tracheids are no 

 longer present. The strand (st') consists only of short, wide vessels, and 

 narrow ones with transverse walls, together with wide, thinner walled wood 

 cells, abutting on each other transversely. 



The single strand in Fig. 184 (st) in the normal wood has divided in 

 the tissue of the callus into two strands (st'), and these again into four 

 strands in the part still further from the cut surface (Fig. 185 st'), at the 

 same time the new bundles are pushed out of their original position by 

 the formation of new medullary rays (Fig. 185 ni). They advance as 

 separate groups toward the periphery of the constantly thickening callus. 

 With the broadening of the tertiary medullary rays these thin vascular 

 strands (Fig. 185 st'), which (in longitudinal section) seem to branch as 

 they growth in length, separate farther and farther from each other until 

 they finally disappear entirely near the outer edge of the callus. The 

 terminals of these strands are short, broad, porous cells of wood parenchyma. 



It is well known that each vascular strand is made up of both phloem 

 and xylem. The wood and phloem are sister elements* In Fig. 184 b, we 

 see a group of wood fibres, which belongs to the xylem strand st ; b' and bb' 

 represent the phloem, belonging to st', the cells of which, analogous to w^ood 

 elements, have become broader. The radial thickening of the phloem cells 

 is not very well shown in the drawing. 



In the fall, when the grapevine has cut off the cortex by a cork zone, 

 the sinuous cork layer (k), in the callus, has divided the phloem bundles 

 into two parts (Fig. 184, b' to bb') ; cV represents in Figs. 184 and 185, the 

 cambial zone. In Fig. 185, is a pouch cell with calcium oxalate in the 

 form of raphides. In some pouch cells sharp, jagged very small protuber- 

 ances project from the inner cell wall. 



The first differentiation in the callus may still be recognized after it has 

 passed over into the finished overgrowth of the callus, beginning at the outer- 

 most cork layer; i. e. if, in Fig. 185, the section begins at the part curling 

 farthest downward and then advances upward. If we designate the part 



1 Ratzebui-g:, Waldverderbnis I, 70. 



