6ii 



B. The annual ring, produced after the frost, is marked /. If this is fol- 

 lowed back to the place of injury, it is seen to diminish suddenly to a point 

 and to be entirely absent under the dried, dead place in the bark {f, t). 

 Only the next annual ring would be able to push between these. The 

 structure of these pointed overgrowth edges resembles much more the 

 normal wood because of the very scantily formed parenchyma wood and 

 the rapidly appearing thick-walled wood cells together with the ducts, than 

 does the lip-like wood parenchyma overgrowth edges of the canker (cf. 

 "Open canker"). 



In the adjoining Fig. 147 

 we see, above the pith bridge 

 (m), the normal annual rings, 

 interrupted by darker, sickel- 

 shaped zones (ps), which here 

 appear gray. These zones con- 

 sist at times of thinner walled, 

 ductless, shortened parenchyma 

 cells, and at times of wood 

 parenchyma, richer in starch. 

 In luxuriantly growing varie- 

 ties the radii of the medullary 

 rays, which here are straight, 

 appear somewhat bent and dis- 

 place the longitudinally elon- 

 gated wood cells and ducts 

 from a diagonal to a horizontal 

 direction. 



It was stated above that the 

 frost plates should be consid- 

 ered as narrowly limited scald 

 injuries of relatively small ex- 

 tent in all directions, which 

 could, however, be found dis- 

 playing all transitions up to 

 large, blasted surfaces cover- 

 ing the whole side of the tree. 

 Besides occurring in pears, 

 such frost plates may also be 



found in the red beech. On branches of a beech thickly covered by such 

 plates, the browning of the contents of individual cells, scattered through 

 the pith, could be proved to be the final radiation of the frost action in its 

 furthest extension into the healthy tissue. These cells undoubtedly have a 

 different content from the other pith cells, which have remained colorless 

 and, in cell contents, probably approach most nearly those of the medullary 

 crown, which likewise easily become brov/n. 



Fig. 147. Cross-section through a pear stem 

 at a blight spot, produced by frost. 



