CONSTRICTION OF TWIGS BY THE BAG WORM. 165 
sycamore and maple under the pressure of the bag worm 
bands. 
Kiister (7), in an investigation on the union of apposed 
branches, states that the first effect of the mutual pressure 
of two such organs, aside from the changed direction of 
the medullary rays, consists in a flattening of the surfaces _ 
apposed, due to growth reduction. An increased growth 
of the primary bark at the edges takes place. He does not 
agree with Krabbe as to the activity of the cambium, find- 
ing that the growth of the cambium does not remain nor- 
mal up to the point of union, but that growth stops entirely, 
because of too great pressure. Before it stops growing, 
Kiister finds that the cambium forms a more or less well 
developed layer of modified woody tissue, which Krabbe 
does not mention at all. This modified layer of woody 
tissue differs from the normal wood in that it is composed 
of parenchymatous cells, and is to be considered as a par- 
encbymatous tissue. It consists of libriform vessels and 
parenchymatous cells, which are more or less thickened, 
pitted and lignified. For the sycamore, Kiister states that 
‘it is surprising to see the extraordinary development 
under the influence of pressure; parenchymatous tissue 
develops, which starts sharply defined from the normal 
wood. Outside of this normal wood one finds a thick layer 
of radially elongated, lignified and very much pitted 
parenchymatous cells. True wood cells continue to be 
formed only here and there, and in a cross-section the 
groups of these wood cells enter into the parenchymatous 
mass in a wedgelike manner. Finally the formation of 
even these isolated groups of wood cells etops entirely. . . 
The pressure is not the physical cause of this growth, 
but acts only as a stimulus, which affects the protoplasm, 
and the real cause must be sought in the nature of the 
latter itself.’’ Kiister discusses the apparent discrepancy 
of his own and Krabbe’s results, and sees an explanation in 
the different manner in which the pressure was applied; in 
