6io 



approach more and more closely to one another from the sides until they 

 finally unite, cover the blackened place in the wood and thus push out the 

 previously attached bark and throw it off. 



In Fig. 146, which represents a blighted young 

 pear trunk, we see at the top, the old, blackened, 

 exposed wood body which originally was covered with 

 bark in a fresh condition ; it is left light in the drawing. 

 The bark on the whole side of the tree has been killed 

 by frost, dried up and thrown off from the healthy 

 [)art by the overgrowth edges which appear after frost. 

 The swollen place at the base of the drawing illustrates 

 the broadening of the flattened trunk, which occurs 

 frequently at blighted places because of the in- 

 creased formation of wood by the uninjured, adjacent 

 tissue. 



On thin twigs, the frost plates are often very 

 small, but the wood under the dried bark is found to 

 be split radially. The cleft, which closes after the 

 abatement of the frost, is now rapidly overgrown ; 

 the dead bark is thrown off at once and the over- 

 growth edges unite. In this, the union takes place 

 after the manner of frost ridges, i. e., the edges rise 

 up like ridges above the normal plane of the annual 

 ring, while the broad wounds which are closed very 

 slowly show the axial cylinder to be flattened at the 

 frozen place. 



In both cases, however, the overgrowth edges are 

 distinguished by the fact that they arise under the high 

 pressure of the dead bark and, on this account, are 

 smallest at the outermost ends and pointed like wedges. 

 This -wedge-like grozvth of the overgrowth edges, 

 which spread out over the dead surface, is a character- 

 istic of Might in contrast to canker. The overgrowth 

 edges of canker increase in thickness towards the place 

 of injury and, like rolls, sink down into the open split 

 ■ivhich forms the beginning of the canker. 



It may easily be seen, that the tissues of the over- 

 growth edges dift'er according to the pressure condi- 

 tions, under which they arise. This has been discussed 

 more in detail under canker. 

 In Fig. 147, the dark place B corresponds to the frost plate p in Fig. 

 145; ? is a piece of dead bark, the healthy part of which {R), recognizable 

 by its white, glistening, hard bast bundles {hh) , is separated from the dead 

 tissue by a diagonal cork zone, adjoining the normal cork covering (/v) at 



Fig-. 146. Young- pear 

 trunk with different 

 kinds of blight spots. 



