CONSTRICTION OF TWIGS BY THE BAG woRM.- 169 
layer of bark the cambium is free to develop without the re- 
straining pressure exercised by the bark, and as a result 
grows rapidly in the direction of the least strain. This 
growth continues until the callous surfaces formed on the 
upper and lower sides of the wound unite and re-establish the 
normal pressure conditions. In ‘the case of the constric- 
tions caused by the bands of the bag worm, a similar dis- 
turbance is brought about by arresting the development of 
the cambial layer for a certain distance completely around 
the twig, and thereby disturbing and probably rendering 
inactive the normal bark pressures. To all intents and 
purposes therefore the formation of the lips of the bag 
worm constrictions may be said to be due to a similar stim- 
ulus to that which results in callous formation after a 
wound, though the formation of the lips is not to be re- 
garded as a callous development, but rather as a new type 
of healing of a different character. 
It is a well-known fact that, where branches are girdled 
by removing a portion of the bark down to the woody 
cylinder, the development of callous tissue takes place from 
the upper or outer edge of the wound. The transfer of 
carbohydrates and nitrogenous substances takes place in a 
downward direction from the leaves towards the trunk ( Cza- 
pek,land2). Some writers, e. g., Fischer (3), have gone 
so far as to maintain that the carbohydrates formed in the 
leaves go downward only in the bark, and that in girdled 
branches they cannot even be transferred across the wound 
through the pith or wood parenchyma. Although very 
little attention has been paid to the problem of nutrition in 
the healing of the disturbance caused by the bands of the 
bag worm, it is nevertheless suggestive to note the differ- 
ence in the manner of healing between the coniferous and 
hardwood trees. As soon as the pressure exerted by the 
bands becomes great enough to obstruct the cells of the 
bark, an evident congestion takes place in the region above 
the band. In the coniferous trees the food supply is evi-' 
