T9I0] PENNINGTON—LONGITUDINAL COMPRESSION 275 
Under such circumstances [when a plant part is compressed] the tension 
of the cell walls is gradually replaced by the external pressure, against which 
the whole osmotic pressure finally acts. 
It is thus seen that the plant does not meet a compressing force 
by growth and cell division, but by osmotic pressure. In reference 
to growth activity in cells under compression, NoLt’s (22) experi- 
ment seems very significant. He showed that in bent roots second- 
ary roots do not arise from the concave side where the cells must 
be compressed. 
Hartic (11), Crestar (5), and SonNTAG (31) attributed the 
increased thickness of cell walls in heartwood to pressure. Now 
it does not seem reasonable to think that pressure could cause the 
walls to become thicker, unless it acts as a stimulus merely to cause 
greater protoplasmic activity. Since the plant opposes external 
compression, or resistance to growth, by its osmotic pressure, it 
would be detrimental to the plant to have the walls lignified before 
their limits of expansion had been reached. That secondary thick- 
ening does not take place until the extension of the cell wall has 
ceased seems to be the general rule. PFEFFER (oP. cit., p.27) says: 
After the stretching growth of the cell has ceased, the cell wall commonly 
undergoes secondary thickening; 
and again (0. cit., p. 32): 
An intimate correlation exists between the different forms of growth, and 
the thickening, cuticularizing, or lignification of the cell wall usually take 
place when the external growth has ceased. 
In regard to the part which the increase in thickness plays in 
overcoming resistance, we may again quote PFEFFER (op. cit., p. 
124): 
Even when external growth has ceased, the thickening of the cell walls 
may still further increase the pressure which the growing tissue is capable of 
exerting. The observed pressures, however, are mot greater than could be 
Produced by the osmotic pressure alone. 
BicHer (4) found in his ‘‘kamptotrophic”’ and “ geotrophic”’ 
Stems that thickening of cell walls was always retarded in the 
parts of the stem which were under compression. 
__ The best experimental evidence of the effect of pressure upon 
_ cell walls was given by NewcomBE (20). Plant stems were inclosed 
