274 BOTANICAL GAZETTE [OCTOBER 
The effect upon the plastic growing cell must be the same 
whether it is compressed or whether it meets resistance to further 
growth. ‘There can also be no difference in the hydrostatic or the 
osmotic pressure within the cell, whether the pressure be in one 
direction or another. In stems a longitudinal pressure causes the 
cells to exert a greater lateral pressure upon each other, and vice 
versa a lateral pressure must cause them to exert a longitudinal 
- pressure upon each other. 
Earlier writers assumed that growth must be inversely propor- 
tional to the external pressure exerted upon the growing cells. 
According to this view, the greatest growth must be in the region 
of the least pressure. It was upon this assumption that DETLEF- 
SEN (7), Sacus (30), NoRDLINGER (23), and Kny (17) sought to 
explain the eccentric growth of stems and branches. They had 
then merely to account for a loosening of the bark in some way 
to cause increased growth. Dr Vries (8) would account for the 
annual rings by assuming that the bark pressure is greater at some 
seasons of the year than at other seasons. The experiments, 1n 
which he retarded growth and caused the production of smaller 
cells by putting ligaments around the trunk, showed that great | 
pressure may retard growth, but his relieving pressure by cutting 
the bark with a knife produced traumatic effects and not true 
growth. Kraspe’s (18) experiments proved that these theorles 
for unequal growth were erroneous, for he showed that the bark 
pressure is never great enough to retard growth. This was an 
indication that growth cannot be directly or indirectly propor 
tional to the pressure. This point was still further emphasized 
and cleared up by Prerrer (25). PrerreR says also (Plant 
Physiology 22125): 
When a root pushes an object in front of it, its rapidity of growth is not 
perceptibly affected unless the resistance offered is very great. 
Although the living protoplasm must control in a large measure 
the manner in which the cell meets external compression, this 
subject is discussed under the head of mechanical effects rather 
than in connection with compression as a stimulus. The manner 
in which the cell meets external resistance is best told by PFEFFER 
(op. cit., p. 124): 
