164 MISSOURI BOTANICAL GARDEN. 
Krabbe (5) disagrees with both Sachs and DeVries as to 
the existence of bark pressure. In a series of extensive 
experiments he shows that the difference in the bark pres- 
sure during the spring and summer is so small that it could 
not possibly account for the difference in structure of 
spring and summer wood. In another series of experi- 
ments Krabbe (6) increased the bark pressure by means of 
- wooden cylinders formed into a chain, to which weights 
were attached; by increasing the pressure on the growing 
tree he found that one can increase the bark pressure two 
or three times without influencing the growth in thickness 
of the branch. For conifers he found that a pressure of 
from three to five atmospheres slowed up the growth and 
changed the radial diameter of the cells, while a pressure 
of ten atmospheres stopped the growth entirely. For 
broad-leaved trees he found that a pressure of from five to 
seven atmospheres reduces growth; but he failed to deter- 
mine the upper limit of growth. In discussing the mor- 
phological changes which the wood cells undergo as the 
pressure increases, he notes in particular a reduction 
in the number of cells formed as the pressure increases, 
and the gradual change of the vessels from an ellip- 
tical cross-section to circular. Another point which 
he notes is that the cambial cells continued to form 
wood cells of a normal character, irrespective of any 
pressure which he was able to bring to bear upon 
them. The newly formed wood cells, which under ordi- 
nary circumstances are capable of expanding until they 
reach a size very much larger than when first formed, are 
materially influenced by the pressure, and as the pressure 
increases, the stretching of the newly formed wood cells 
may cease altogether, thereby giving rise toa much smaller 
Jumen than is normally the case. Although Krabbe states 
nothing as to the shifting of the medullary rays, he shows 
(in two figures) a shifting similar to that found in the 
