6i7 



already observed in the normal wood formation a ring-like break in the 

 form of a line of reddish tissue. Corresponding reports and observations 

 may be found elsewhere which, however, do not contain any new points of 

 view. Studies on canker phenomena increased our understanding of the 

 disturbances in the formation of annual rings. I have proved in the apple 

 canker that an annual ring, which is simple and normal on the healthy side 

 of the branch, may be subdivided on the canker side into several ring zones. 

 My recent studies on the oak have shown how such a breaking up of the 

 tissue may take place. 

 Experimental Production of Parenchyma Wood by Frost Action. 



The cases of the production of parenchymatous wood tissue instead of 

 normal parenchyma, described in the preceding chapter as "pith spots," 

 "parenchyma wood bands," "ring shells," etc., arise from a variety of causes 

 which, however, as a whole, agree, in that the cambium in different parts of, 

 or to the whole extent of the annual ring, is more or less freed from the 

 pressure of the bark girdle binding it. It may be concluded from subse- 

 quent observations that frost, and especially spring frosts, furnish one of 

 the most essential and frequent causes of such a loosening of the bark girdle. 



In 1904, in May, a frost had so greatly injured the younger oak shoots 

 near the edges of different forest plantations, where these bordered on open 

 meadows, that a number of branch tips were completely frozen while only 

 the leaves of others had blackened and dried; later they continued their 

 growth at the tips. When these shoots, within a few weeks, had again 

 formed new leaves, they were cut for investigation. They showed great 

 dift'erences in structure, among others that illustrated in Fig. 148. 



We recognize an irregularly pentagonal medullary body (ni) sur- 

 rounded by slender wood rings (h) more strongly developed on one side. 

 This wood ring, however, on the outside, does not adjoin a regular cambial 

 zone, as is the case in the normal branch, but passes over suddenly into a 

 porous, wide-celled parenchyma wood (ph) which becomes thicker walled 

 toward the bark and only rarely leaves recognizable a cambial boundar}' 

 zone between itself and the bark. That this girdle (ph) formed of porous 

 tissue still belongs to the wood ring and has arisen from it, is proved by the 

 short-celled, vascular elements (g') scattered in the zone of thin-walled 

 cells which, in the structure of their thickening layers, seem similar to 

 those of the ducts in the normal, first formed wood ring, or resemble them. 

 This presence of short ducts, or duct cells, and the condensing of the whole 

 zone of thin-walled cells at its periphery by the occurrence of thick-walled 

 elements, resembling the true wood cells, shows, therefore, that this branch, 

 injured by frost, had re-adapted itself to the normal formation of the wood 

 ring a short time after the cessation of the frost action and the formation 

 of the parenchyma wood. 



If this bra.nch had been allowed to continue growth until frost, we 

 would then have had a second false annual ring, as has been observed by 

 earlier investigators and was discussed in the preceding chapter. 



