585 



This tissue has been killed b)- frost and torn by the subsequent drying. 

 The parenchyma, lying in the direction z'-i'o, had not been formed at the 

 time of the frost action (May i8th), but the splitting of the medullary 

 bridge was extended outward through the bark. The bark in the cambial 

 zone at that time was also split away from the sapwood at both sides and 

 formed the split (s p) but only the cells lying directly on the edges of the 

 wound had died and partially dried. The two sides of the bark above the 

 split (s p), which originally had been separated, at once formed the initial 

 stages of the overgrowth edges in the manner common to all processes of 

 circumvallation by the outcurxing of the peripheral healthy cells and their 

 division. These o\ergrowt]i edges are formed further and further out 

 toward one another until in a short time they coalesce. 



The place of coalescence of the circumvallation edges (n r) may be 

 recognized by the diseased depression {v a), but especially by the position 

 of the hard bast cells (b). which seem inclined toward one another. The 

 whole tissue, which covers the split, has been formed anew in the course 

 of six weeks (the wound was investigated on the 4th of July). The old 

 bark, which had been split by the frost tear, is pressed back by the lip-like, 

 protruding circumvallation edges and now surrounds the new structure 

 like a sharp edge (t). The circumvallation edge at this time had formed 

 wood. The whole thick-walled zone (hp) is new wood. This, however, 

 has been produced with so little bark pressure that it has become parenchy- 

 matous and short celled. Only later did the cambial zone (c-c), produced 

 by the coalescence of the zones, which had been isolated in two halves, form 

 normal wood elements and deposit firmer layers about the frost wound. 



Similar to this injury to the larch is a wound produced on an apple 

 branch by the action of cold at 3 degrees C. which lasted for 25 minutes 

 in July (Fig. 132). In this, a indicates the old wood of the previous year; 

 /;, new wood formed up to July; c, the region in which the cold had killed 

 the tissue. In the very luxuriant overgrowth edges, extending above the 

 surface of the wound, the spirally curved cambial zone (/) has produced 

 a thick new bark {g ) and a new wood body (e), divided radially by the 

 medullary rays (d). But this formation of wood from prosenchymatous 

 elements begins first rather far back in the circumvallation edge. The lip- 

 like part of the edge, lying in front of it, consists of parenchyma wood, on 

 the edge of which may be recognized gradually dififering prosenchymatous 

 cell groups (h). In the same radius, in which the first thick- walled wood 

 cells occur, the beginnings of the hard bast cells (h b) appear in the bark. 



The circumvallation edges extend over the bark as a knob with a lip- 

 like cleft. This appearance is retained because of the natural swellings 

 which are met with at times in the branches from cankered trunks of 

 apple, beech, ash and cherry trees, and which I consider to be the initial 

 stages of the closed canker swelling (cf. Fig. 135 in the ^following 

 section). 



