BOTANY: W. J. V. OSTERHOUT 
239 
surface layers of protoplasmic structures behave in this way.* If there- 
fore, such structures exist within the cell, it is evident that any deforma- 
tion of the protoplasm which is sufficient to rupture their surface layers 
will permit their contents to diffuse out into the surrounding protoplasm. 
A great variety of cellular structures (plastids, vacuoles, 'microsomes,' 
inclusions, etc.), possess surface layers of great delicacy and it is easy to 
see how some of these may be ruptured by even the sHghtest mechanical 
disturbance. 
It is therefore evident that deformation of the protoplasm may rup- 
ture the surface layers of certain protoplasmic structures and cause 
their contents to diffuse out. If the substances which thus diffuse out 
meet other substances from which they were separated by the semi- 
permeable surface layer before it was ruptured it is easy to see how 
reactions may be set up which in certain cells may bring about the 
responses characteristic of mechanical stimulation. The occurrence of 
such reactions seems probable, since many cases are known where sub- 
stances in close juxtaposition are prevented from reacting by the pres- 
ence of such semipermeable layers ; but when these layers are destroyed 
(by crushing the cells) the reaction at once takes place. 
If these processes occur it is evident that purely physical alterations 
in the protoplasm can give rise to chemical changes. Responses to 
contact and mechanical stimuH may thus be explained; and since gravi- 
tational stimuli involve deformation of the protoplasm we may extend 
this conception to geotropism. 
In this conception of mechanical stimulation the essential things are 
(1) substances which are more or less completely prevented from react- 
ing by semipermeable surfaces, (2) a deformation of the protoplasm 
sufhcient to produce in some of these surfaces a rupture which is not at 
once repaired, (3) a resulting reaction which produces the characteristic 
response to the stimulus. 
* In many cases rupture of the plasma membrane causes the protoplasm to disintegrate 
and mix with the surrounding medium. In other cases the surface layer is at once re- 
constituted. 
