126 



PROPERTIES OF WOOD. 





outpouring of resin into the cambium zone in early spring 

 after only a few spring cells of wood had been formed, the 

 mass of resin was isolated by occluding tissue. Tschirch 

 suggested that this is due to a wound, but if so, why, always 

 at a definite season of the year and near the crown of the tree 



as well as low down the stem ? 

 The wound may, however, be 

 caused by an insect. The pre- 

 valent opinion that resin-galls 

 are due to a conversion of the 

 cell-walls into resin has been 

 proved by Mayr to be wrong. 

 Kesin-galls occur in coniferous 

 woods that have resin-ducts, 

 spruces, pines, larches and 

 Douglas-fir ; as they are patho- 

 logical, nothing can be decided 

 about the identity of woods by 

 their absence, but if present 

 they distinctly prove that the 

 wood is not that of silver-firs, 

 tsugas or cypresses. The wood 

 of the latter trees, however, 

 sometimes has rudimentary 

 resin-ducts, especially on wood 

 occluding wounds caused by 

 hail or frost. In wood with 

 resin-ducts there may be 

 abnormal numbers of them in certain decades and in others 

 only a few. 



Gall-parenchyma, especially in broadleaved trees is pro- 

 duced by the stimulation of insects and their larvae Species 

 of Lachnus, a kind of aphis, form a gall on forest plants in 

 which all stages of occluding tissue, from parenchyma to 

 wood-tissue, occur. 



Abnormal cell-formations resembling occluding tissue are 

 caused by late frost in both broadleaved and coniferous wood. 

 Damage done by late frost to the cambium is often 

 accompanied, in lengthening shoots, by a bending of the shoot 



Fig. 46. Longitudinal section 

 through a resin-gall in spruce- 

 wood. Occluding tissue in little 

 semi-circular groups of cells pro- 

 truding into the hollow filled 

 with resin. The external zones 

 are convex. 



