HooKER: MOVEMENT IN DROSERA ROTUNDIFOLIA 399 
3. ALTERATION OF CELL-WALL ELASTICITY 
In the opinion of Noll (’95, p. 65) the change in the plasticity 
of the cell-wall is analogous to the gradual change that takes place 
in a strung bow, which after having been left strung a long time 
does not return completely to its original shape when unstrung. 
This plastic change does not involve any permanent diminution 
of elasticity, however, for the bow returns to its new shape after 
bending. The source of energy for this change in plasticity is 
found in the stored energy of elastic tension. The change from a 
condition of elastic tension to a plastic alteration of shape is 
compared by Noll (’95, pp. 79-81) to changes produced in rubber 
by vulcanization, during which process any elastic deformations 
that happen to be present are transformed in a greater or less 
degree to permanent alterations of shape. In bending plant 
organs he assumes that the protoplasm secretes one or more 
substances which act on the cell-wall as vulcanizing sulphur acts 
on crude rubber. 
The correlation between the molecular structure and the 
physical properties of metals, which has been discovered in recent 
years by metallographists, offers a more satisfactory interpretation 
of the changes in the physical properties of the cell-wall. The 
alteration of the strength, ductility and elasticity of metals pro- 
duced by mechanical treatment and by heat are based on changes 
in molecular structure, which consist in the rearrangement of the 
relative amounts of the phases that enter into the composition of 
the metal. Two phases are usually present, a crystalline phase 
and an amorphous phase that cements the crystals together. The 
_ strength and ductility of a metal are found to be increased and the 
elasticity diminished by an increase in the amount of the amor- 
phous phase and a decrease in the amount of the crystalline phase. 
Conversely the elasticity would be increased and the strength and 
ductility diminished by an augmentation of the crystalline phase 
and a decrease in the amorphous phase. It is probable that 
changes in the elasticity of plant cell-walls are also produced by 
alterations in the molecular structure. The cell-wall is undoubt- 
edly a mixture of phases in heterogeneous equilibrium. It seems 
plausible that changes in elasticity are effected by a readjustment 
between these phases. Possibly an amorphous and a crystalline 
