AN ODD FORM OF HYPERTROPHY IN ARBOR VITAE 



BY HOMER D. HOUSE 



Plate B. 



Hypertrophic growths on the trunks or limbs of trees in the 

 form of burls, abnormal accretions, etc, are familiar sights to every- 

 one who gives any attention to trees. Rarely, however, is there 

 seen so remarkable a case of hypertrophy as is shown in the accom- 

 panying illustration, which was taken from an Arbor Vitae 

 (Thuya occidentalis) in Canada, by Dr J. M. Clarke. 

 Director of the State Museum. 



Various causes are ascribed to hypertrophic growths, the most 

 usual being: 



a Injury to the inner bark or cambium resulting in a stimulated 

 production of abnormal tissue, the abnormal growth once started 

 continuing to produce successive layers about its core. 



b Insect or fungus attack, stimulating the growing tissues into 

 abnormal growth at or about the affected portions. 



c Parasitic mistletoe attack, causing hypertrophy of the affected 

 tissues. 



Any of these causes might be suspected as the origin of certain 

 malformations of wood and bark upon trees. In the absence, how- 

 ever, of definite evidence, which is the usual case, such growths 

 might be due to other agencies, purely physiological in character, 

 which might cause a disarrangement of the normal growth of the 

 tissues. Practically all observations, however, seem to agree on the 

 initial presence of some sort of injury, preceding the hypertrophic 

 growth, evidence of which can sometimes be detected at the origin of 

 the growth. 



In the case of the hypertrophic growth here shown there is 

 little evidence as to its cause. The grain of the wood within 

 the trunk and beneath the seat of the growth is straight and 

 normal. About 74 annual rings of growth (less than two inches 

 of wood) intervene between the origin of the hypertrophic growth 

 within the trunk and the present surface of the bark. A marked 

 cup-shaped depression in the fiber of the trunk shows where 

 the growth apparently originated, and it is evident that this ab- 

 normal growth on the outside of the trunk lias obtained its present 

 dimensions of over two feet in diameter within the last 75 years. 

 Slight traces of decay only are apparent in the fiber of the trunk 

 close to the central core of the hypertrophic growth. 



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