793 



only a few peripheral cell rows always remain free from starch and free 

 from subsequently formed calcium oxalate, for the tissue which extends 

 beyond the cut surface, and which warrants the name "callus" only so long 

 as it is absolutely undifferentiated, soon shows a difference in its structure 

 and passes very rapidly from the callus stage into that of the overgrowth 

 edge. Soon after the formation of the peripheral cork covering, a meristem 

 zone appears also in the interior of the callus tissue and represents the 

 continuation of the cambial ring of the normal piece of the vine within the 

 overgrowth edge. Besides this meristematic zone, the first traces of a bast 

 body may also be recognized in the separated parenchymatous cells lying 

 scattered close under the cork zone. These cells appear to have somewhat 

 more strongly refractive, easily swelling walls (&"'). In some of these I 

 think I have recognized indications of sieve pores similar to those found in 

 the tangential walls of normal bark sieve cells (ss), so that the conclusion 

 may be drawn that the first differentiation of the callus tissue, appearing 

 almost simultaneously with the formation of the new cambial zone, consists 

 in the formation of sieve cells within the bark. 



The tissue formed in the cambial zone appears, in Fig. 183, to be divided 

 longitudinally by the medullary ray cells (w). These are elongated radially, 

 have clearer contents and like the rest of the tissue are small celled at the 

 periphery of the overgrowth edge. Their approximately perpendicular 

 direction changes gradually into the normal horizontal one as the rays 

 extend into the normal tissue of the uninjured piece of the vine. 



In the youngest portion of the callus edges, where the tissue lying next 

 the cork border first arose, one finds the wood lying between the clearer 

 medullary rays to be short, thin-walled and parenchymatous. The further 

 the wood is examined back toward the normal tissue, the longer and thicker 

 walled it appears and it passes from its radial direction more and more into 

 the longitudinal elongation of the normal wood elements. The earlier in 

 the year the girdling is undertaken, i. e. the longer the newly produced 

 cambial zone of the overgrowth wall produces wood, so much the more do 

 the later formed elements approach normal wood in length and form. 



Scalariform vessels (g,s) appear in this thin-walled parenchymatous 

 wood as the first thick-walled elements ; they have at first the size and 

 arrangement of the wood parenchyma cells of the surrounding tissue but 

 assume gradually the form and arrangement of normal vessels the nearer 

 they approach the uninjured parts of the wood. In opposition to de Vries, I 

 must maintain that the short duct cells are not always the first formed thick- 

 walled elements. When the callus at the lower margin of a girdle is very 

 weakly developed, the wood parenchyma often passes over directly into 

 normally arranged, slightly thickened xylem, elements, without the previous 

 appearance of short duct cells. 



In the callus at the upper margin of a girdle which in the same length 

 of time has developed more than twice as extensively as the lower callus, 

 the cambial zone is broader, all the elements are more numerous and the 



