802 



with its vessels (v). A to A', according to Trecul, is the old bark of the 

 previous year^ The split, which pushed up the bark, extends horizontally 

 above the highest vessel {v) to the point marked x' ; from there it runs 

 downward toward the right almost to the thin-walled, last-formed cells of 

 the previous year, so that the whole group {g) should be considered as a 

 new structure. At x, the loosened bark has removed only the outermost 

 layers of the youngest wood, or has possibly extended only to the central 

 cambial zone, so that the whole sapwood has remained in place. The 

 outermost cells elongate (/) and divide (/'). The upper cell (r) of each 

 row, cut ofif by the new wall, repeats the process. 



Fig. 186. Callus formaton from young bark cells in a barked trunk. (After Trecul.) 



The young wood (sapwood) has been stretched radially by the injury, 

 i. e. by the removal of the bark pressure. It forms shorter cells with wider 

 lumens but has remained thin-walled, while the vessels already started have 

 matured. 



From x' out, the young sapwood has been removed with the loosened 

 bark and on the wood of the previous year only a few young wood cells of 

 the current year were left. These cells have now taken over the formation 



1 It might seem strange that the annual ring at A' ends with a very thin- 

 walled spring wood, but such cases actually occur. I obtained from the Eifel in 

 January larches diseased with canker, the annual ring of which had formed over 

 the summer wood a layer six cells thick of thin-walled spring wood. 



