536 
ALBAN STEWART 
with one of the layers just mentioned. In the early stages of develop- 
ment of the gall this layered arrangement of the stone cells is not 
greatly interfered with, but in later stages the layers are broken up 
resulting in irregular masses scattered through the bark which are 
composed of stone cells larger than those normally present. The lumina 
of many of these contain a yellowish-brown substance which gives a 
strong reaction for tannin. They are evidently the same as the tannin 
balls described by Hartwich (lo, pp. 147-149). Similar tannin 
accumulations also occur in other cells of the bark as well as in the 
cells of the protecting layer. 
Corky inclusions occur quite commonly in the bark of the gall, 
which surround either groups of parenchyma or stone cells. One of 
these bodies is shown in fig. 7, where the cork forms a light colored 
circular figure at the center of the photograph. Maule (15, p. 24) 
mentions cork growths in the bark of wounded stems but in these 
instances the cork is surrounded by a woody mantle composed of 
wood parenchyma cells and short tracheids. Groups of irregularly 
arranged tracheal elements also occur in the bark of the gall, the 
tracheids which compose them being short, sometimes nearly iso- 
diametric, with scalariform markings on their walls. These may be 
arranged as single strands, or groups of tracheids may be associated 
together which are connected with similar groups a short distance 
away by cross bridging. The arrangement of the elements is often 
very complex resulting in twisted masses of tracheids as is shown in 
the center of fig. 6. Parenchyma cells sometimes occur in these 
tracheal complexes in the bark, the arrangement of which suggests 
narrow rays. Somewhat similar bodies are found in the bark of the 
black knot gall on Prunus virginiana (see Stewart 20, p. 120). Small 
groups of parenchyma cells may become partially surrounded by a 
woody growth in such a way as superficially to resemble the cross 
section of a sector of a small dicotyledonous stem, the parenchyma 
around which the woody growth takes place corresponding in position 
to the pith of such a stem. Some of these woody bodies have a 
cambium and seem to be capable of further growth. 
But little is known about the origin of these peculiar woody bodies 
which occur in the bark. Maule (15, p. 24) found special ball-forma- 
tions^ and other woody growths in the bark of wounded stems but 
2 The term "Knauel," used in the German literature to designate these pecuHar 
woody growths, is rather difficult properly to express in English. Miss Frances 
