583 



from the wood. Tlie line of separation extends irregularly, sometimes in 

 the cambial layers of the bark, sometimes into those of the sap-wood. Since 

 it can be assumed that in all parts of the tissue of the line of separation an 

 equally strong strain was active in producing the tear, it is evident, from 

 the irregularity of the line of separation, that the tissue at the same radial 

 distance from the centre of the branch does not possess throughout the 

 same firmness. Such an irregularity is shown by the tissue fragments {k) 

 which, remaining attached to the sapwood, die later and are indicated at 

 the side of the projecting wood (/), 



^^'ith the exception of these fragments very little collapsed tissue is 

 found at the torn place, even the cells of the youngest bark (w), which 

 have turned a deep brown and become poor in contents, have not collapsed. 

 Instead they have become stiff and their walls (/") much more resistant to 

 sulfuric acid. 



The healing of such wounds does not as a rule take place by lateral 

 circumvallation. Rather, in similar places, a radial stretching of the older 

 cambial parenchyma is observed at first. Later isolated meristemic aggre- 

 gations are produced in the bark between the bark rays which form new 

 wood elements. The new wood gradually presses the tissue layers (m) 

 which, in this case, have not been changed, against the split sapwood in the 

 direction /, o, e, and forms from the dead tissue remnants a brown stripe 

 which becomes narrower with a greater wood accumulation abov^e the place 

 of rupture, i. e., greater pressure. The isolated meristemic zone of the 

 wood bundles, produced in the raised pieces of the bark, later unite 

 laterally with one another and, finally, with the cambial zone (/), pro- 

 duced on all sides of the still uninjured branch. Such a blister, produced 

 by tangential raising and radial splitting of the wood ring, may remain 

 recognizable externally for. many years. 



Open Frost Tears. 



An apparently very unessential phenomenon, to be found most easilv 

 in vigorously growing nursery specimens, is tlie occurrence of small tears 

 which have been overgrown. These extend also, more or less like blisters, 

 above the smooth bark but are distinguished from those already described 

 in that they have a long grove on their upper surface. From this it is 

 evident that they have been produced by the coalescence of the two edges 

 of wounds which have pushed forward like lips. These elevations grow 

 less and less conspicuous with later growth and finally have no further sig- 

 nificance for the life of the trunk. 



These are. however, of uncommon theoretic importance in explaining 

 the production of the tissue excrescences described later as frost canker. 

 So far as my investigations ha\e gone, they support the theory that the 

 swellings of frost canker have their origin in such small tears as are pro- 

 duced in the spring at the time of the most luxuriant cambial activity of 

 the trunk. .Such tears are found usually in the immediate proximity of the 



