CONSTRICTION OF TWIGS BY THE BAG WORM AND INCI- 
DENT EVIDENCE OF GROWTH PRESSURE.* © 
BY HERMANN VON SCHRENKE. 
INTRODUCTION. 
For a number of years many branches of arbor vitae 
(Thuja occidentalis) in the Missouri Botanical Garden 
showed signs of disease and gradual dying from the tips of 
the branches inward. A careful examination of the twigs 
showed a condition represented on plate 20, figs. 1, 2 and 
3. Ata short distance from the end of the twig a swell- 
ing was usually found, which in the case of twigs which 
were still comparatively green was very small, but in many 
cases reached a size of four and five times the diameter of 
the twig below the swelling. Thepartof the branch above 
the swelling was usually very much larger in diameter than 
‘the branch below the swelling. Where the swelling was at 
all large the parts of the beanah towards the outside of the 
tree were usually found either in a dead or dying condition. 
In every case the diseased branches showed marked ten- 
dencies towards the formation of secondary buds and 
branches, so that frequently the tip of a branch had the 
appearance of a small broom (pl. 20, fig. 3). After look- 
ing at a number of trees the swelling was finally found to 
be due to the girdling action of the bands of the common 
bag worm (Thyridopteryx ephemeraeformis Haworth), 
ey 20, fig. 2). A great many instances of the swellings 
with the bag worm still in position were found in the vi- 
cinity of St. “Louis and in other parts of the country; and 
in all of the older swellings where no bag was visible, the 
band of the worm was found still encircling the branch. 
* Published by permission of the Secretary of Agriculture. The main 
results here detailed were communicated to the Society for Plant Mor- 
‘ phology and Physiology, at its Ann Arbor Meeting, in December, 1905. 
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