6i3 



cells, extending in the form of an arch from bark ray to bark ray, and 

 already elongated, suffer less than the inner, small celled tissue which 

 bounds them. 



The observations, here mentioned, illustrate frequent, isolated cases 

 but not phenomena of universal occurrence. Finally, a case in the sweet 

 cherry should be mentioned as especially noteworthy. The pith of a one 

 year old branch seemed spHt at one side up to and beyond the centre and 

 the cells of the periphery of the pith grew out like filaments into the result- 

 ing cavity, similar to the woolly stripes of the apple core. No gummosis 

 was present. The case was observ^ed in the so-called "frost-wrinkles." It 

 is interesting because it shows that the activity of growth in the pith, which 

 in general occurs only in soft wood trees (Tilia), can be reawakened here. 



In the above mentioned phenomena of scald is also found, as a rule, 

 an increase of the gum centres in the Amygdalaceae and of the resin centres 

 in conifers, with an increase of the parenchyma masses (Fig. 147 pz) be- 

 tween the normal parts of the annual ring, just as in canker. In canker, 

 it can also be proved that the breaking up of the bark due to a weakening 

 of the mechanical ring corresponds to the breaking up of the wood by 

 parenchyma wood in the same radius. The hard bast bundles are absent 

 from the bark of the overgrowth edges just as are the real, thick-walled 

 wood cells in the wood of these edges. 



Aggregations of Parenchyma Wood. 



In canker excrescences, we have seen how tender and perishable the 

 wood ring becomes as soon as it passes over into the overgrowth edges of 

 a narrow cleft at the time of the greatest growth activity in the spring. 

 Because of the rapidity of the production of such large tissue masses, the 

 wood ring does not have time to mature prosenchymatous elements but at 

 first is formed of parenchymatous, thin- walled elements which, to be sure, 

 have some advantages as a storage tissue for reserve substances, but show 

 very slight power of resistance to parasitic and atmospheric influences. It 

 is therefore easily understandable that even in healthy trees the appearance 

 of parenchyma, instead of prosenchyma wood, deserves especial attention 

 from a pathological standpoint. Such cases may be found everywhere. 



The aggregations of parenchyma wood can occur in the trunk in the 

 form of scattered nests, or in ring-like bands, differing in length and width. 

 They have been variously named. We find an enumeration of such cases 

 in de Bary^ , who sees in them an hypertrophy of the medullary rays. 

 Rossmassler calls them "Repetitions of the pith;" Nordlinger, "pith spots," 

 while Th. Hartig- speaks of "cell passages." The most mature form is 

 found in the so-called "moon rings." These are brown, or white, bands of 

 parenchyma wood, usually extending in a ring partially or entirely around 

 the trunk. This parenchyma wood appears at times to be decayed like 



1 De Bary, Vergleichende Anatomie der Vegetationsorgans 1877, p,. 567. 



2 Hartig-, Th., Vollstandige Naturgeschichte der forstlichen Kulturpflanzen. 

 1852, p. 211. 



