794 



beginnings of the vascular bundles in the callus always start with duct cells. 

 The formation of these cells takes place the earlier the nearer to the old 



wood they are formed. Their form, 

 size, thickness of wall and arrange- 

 ment will be more nearly normal the 

 further back the tissue lies from the 

 cut surface. The vascular strand {g,2) 

 of this tissue grades gradually into the 

 normal wood formed before girdling, 

 thereby forming a pseudo-secondary 

 growth in that area. 



According to the anatomical condi- 

 tions shown in Fig. 183, we may say 

 that the girdling has produced an un- 

 usual loosening of the wood in the 

 uninjured part of the vine adjacent to 

 the wound. In this way the vascular 

 bundles, which are formed of vessels 

 and thick-walled tracheids on one side 

 of the cambium and of the thick-walled 

 phloem fibres and sieve tubes on the 

 other, and which, in normal wood, are 

 arranged close against one another in 

 concentric circles, are separated and 

 broken up into single strands by masses 

 of parenchyma. These strands, g,3 

 (vascular strands), and b' (phloem 

 strands), the elements of which con- 

 stantly become fewer in number, change 

 constantly and continue into the callus, 

 which is gradually covering the girdle. 



We may best see by means of cross 

 sections taken at different heights 

 through the callus, what happens to the 

 vascular cylinder which in the unin- 

 jured portion of the vine consists of 

 the wood and the phloem rings, only. 

 slightly broken by few-celled medullary 

 rays. This cylinder finally is separated 

 into single strands by the growth of 

 parenchyma induced by the girdling. 

 The strands gradually become narrower 

 as they pass outward radially and tan- 

 gentially in wavy lines, they are at first distinct, but later anastamose forming 

 a net and finally split up into isolated strands arranged in fans. 



Fig. 184. Cross-section tlirough a ring- 

 ing roll close to the point where it 

 appears on the plane S to S' in Fig. 183. 



