TRANSACTIONS OF SECTION K. 773 
The Position of Maximum Stimulation. 
This problem involves the question whether an orthotropic organ in the 
vertical position is or is not freed from stimulus. We will first take the question 
as to the existence of a stimulus in the normal (?.e., not the inverted) position. 
One of Pfeffer’s! arguments for the existence of a stimulus is as follows. A root 
haying been allowed to curve from the horizontal to the vertical position is placed 
on a klinostat, and after a time the curve disappears. It is therefore assumed that 
there existed a geotropic stimulus keeping the root curved until the stimulus in 
question was rendered inoperative by the klinostat, when the rectipetality of the 
root could have free play. But it is not a necessary conclusion that while the 
root is strictly vertical any stimulus is acting. If from some internal cause the 
root leaves the vertical, the ordinary geotropic curvature depending on the stimula- 
tion of the tangential walls will come into action and bring the root back to the 
vertical, To translate into the language of the statolith theory, it is not necessary 
to assume that the lower walls of the graviperceptive cells are sensitive to the 
pressure of the statoliths—the sensitiveness of the tangential walls will suffice. The 
experiment above mentioned does not therefore seem to prove that an orthotropic 
organ in stable equilibrium is stimulated. But it is quite conceivable that a 
stimulus might be originated by the Joss of pressure on the lower wall, for this 
would be a well-marked change in the internal condition of the cell, and therefore 
might become associated with a reflex. ‘Thus, when an organ is placed 
horizontal the stimulus from the pressure of statoliths on the lateral walls (now 
horizontal) may be combined with, or in some way influenced by, the loss of 
pressure on the terminal wall of the cell which was formerly horizontal. But if 
the absence of pressure on a cell-wall acts in this way are we not bound to 
consider the pressure (when present) as a stimulus? I think we are, and there- 
fore, though I do not think that the particular experiment referred to supplies the 
necessary evidence, I hold the lower wall of an orthotropic cell to be sensitive to the 
stimulus of statoliths, though such stimulus cannot be of a directive nature, 
Since an organ when accurately inverted? and prevented from circumnutating 
receives no impulse to curve, it is assumed that the normally upper cell-wall 
(which is now below) is not stimulated. According to the statolith theory it is 
inconceivable that the organ should curve, since uniform pressure on the hori- 
‘eg terminal wall cannot determine the direction in which such curve shall 
in. 
a But though no directive stimulus seems to be a possible result of uniform pres- 
sure on the end-walls, it does not follow that such pressure has no effect. It seems 
to me that such a striking change as pressure on a wall which under normal cir- 
cumstances does not receive pressure may very well modify the result of the 
normal stimulation of the lateral walls of the cell. 
Czapek*® has shown that with both stems and roots the gravistimulus is 
greater when the organ is removed from the normal vertical position by 185° than 
when it deviates from the normal by 45°. In the case of an apogeotropic shoot 
the position of the starch in the endoderm is given in fig. 8. The pressure of the 
starch on the lateral walls is the same in the two cases. In i., however, the starch 
rests partly on the basal wall (B), while in ii. it rests, to the same degree, on the 
apical wall (A). On the usual assumption that the basal and apical walls are 
insensitive, there is nothing to differentiate i. from ii. I cannot help suspecting 
that the pressure on the apical wall does in some way affect the sensitiveness of 
the tangential walls. If the pressure on the wall (A) was in itself the decisive 
' Pfeffer (93, p. 19). I am only concerned with this special point, not with 
Pfeffer’s general argument. 
2 In the whole of this discussion the organs are supposed to be supported by 
the morphological base. 
3 Czapek (95, 1). As doubt has been expressed as to the actual facts, it is worth 
while mentioning that Miss Pertz (99) has confirmed his results for the haulms of 
grasses, : 
