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 
Piiaya 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 has 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. 
[45] 
