772 REPORT—1904. 
klinostat. This can best be accounted for, as Noll! suggests, by the supposition 
that the equally distributed stimulus tends to produce a simultaneous increase and 
decrease of growth-rate on opposite sides of the rotating plant.” We, therefore, 
get in an indirect way evidence in favour of what has not been directly proved— 
namely, that in geotropie curvature the diminution of growth on the concave side 
is not the result of compression produced by increased growth on the convex side, 
but rather an independent reaction. It is necessary, therefore, to inquire what. 
theoretical conclusions may be fairly made as to the stimulation correlated with 
such a mechanism of curvature. Noll* uses the term ‘ Reizfeld, or ‘ stimulation- 
area,’ to express the regions in which graviperception occurs. The distribution 
of these areas is expressed in diagrams which serve as shorthand methods of 
recording the geotropic reactions of various organs. All such ways of clarifying 
and expressing our ideas of the laws of perception are useful. I must confess that 
I do not find Noll’s terminology easy to use, and I prefer to express the same 
ideas in terms of the distribution of the pressure of statoliths on the different 
parts of the ectoplasm of the gravisensitive cells. 
Imagine an apogeotropic shoot placed in the horizontal position as shown in 
longitudinal radial section in fig. 2, where C and C’ are the cortical tissues and 
Fie. 2. 
the seat of motile power; E and i’ the endodermis, the supposed region of gravi- 
perception ; M, the central tissues, which do not concern us. 
The fact that the statoliths now rest on the horizontal (tangential) walls difter- 
entiates the horizontal from the vertical position of stable equilibrium. But what 
circumstance is there that can be conceived to originate curvature in one direction 
more than another? It can only be that in the endodermis E on the physically upper 
side the statoliths rest on the inner tangential wall, whereas in E’ they rest in the 
outer wall, This view agrees with Noll’s hypothesis of the arrangement of stimu- 
lation-areas, There is no difficulty in believing that the inner and outer 
tangential walls have different individualities ; Véchting’s work * on transplantation 
seems to indicate that this is the case. And if this analogy with formative 
polarity is not allowable, we must still insist that the presumption is in 
favour of E and E’ in fig. 2 being in different conditions, since we have certainly 
no right to assume that the outer and inner walls are identical in what we have 
called their individuality. 
It is not here necessary to go into the question whether the radial walls of the 
endodermis are or are not sensitive, since the problem of geotropism in its broad 
outlines is not concerned with it.° 
1 Noll (92. p. 35). 
2 We have shown (Darwin and Pertz, 04) that in Se¢aria the statoliths undergo 
changes of position on the klinostat, indicating a succession of stimuli, See Heine 
(35), who briefly describes similar changes. 
3 Noll (92, p. 19), 
* Vochting (92, p. 151). 
5 See the discussion in Haberlandt (03, p, 467). 
