70 
Francis Darwin. 
two supposed points. The validity of this theory not being easily 
the longitudinal walls (now horizontal) would he different at the 
tested, we may go on to the well-known statolith theory. The 
essence of this hypothesis is the conception that the stimulus is 
due to the differences in specific gravity of certain parts of the 
cell. The special form of the statolith theory which is connected 
with the names of Haberlandt 1 and Nemec is that a specialised 
form of starch grain supplies the heavy bodies in question. These 
“ statoliths ” lie freely in the cell and gravitate towards the physi¬ 
cally lowest region. Thus in a vertical cell the pressure of the 
statoliths will be on the basal wall. When the cell is placed 
horizontally the starch will fall away from the basal wall (which 
is now vertical) and spread out in the lower lateral wall. In 
this way it is imagined that the pressure of starch grains on 
different parts of the cell-walls serve as signals informing the 
plant as to its angular position in space. Nor is this a vain 
imagining, because it has been absolutely demonstrated that in the 
case of the Crustacean Palcemon the power of orientation in 
regard to the vertical is dependent on the pressure of the statoliths 
(or otoliths) on the internal surface of the otocyst. This is a 
point of some importance, viz., that the method of orientation 
believed to occur in plants does undoubtedly occur in animals. 
The truth of the statolith theory is not easy to prove or disprove. 
One obvious method is to inquire into the distribution of the above 
described free or “ falling ” starch. Our theory suggests a function 
for the statolith-starch that occurs in the endodermis, and it is 
significant that starch is only to be found here so long as the organ 
is growing, that is to say, so long as it is capable of geotropic 
curvature. It is especially striking that in Monocotyledons, in 
whose leaves starch does not occur, have falling starch in the 
endodermis. 
Then again the occurrence of falling starch in the cotyledons 
and not in the hypocotyl of Sorghum, Setaria, &c., agrees with the 
supposed distribution of geoperception in these seedlings. In the 
case of roots, the fact that there is a special patch of statocytes 2 in 
the root-cap appears at first sight to be in harmony with the localisa¬ 
tion of geoperception in the root, but as I shall show there are some 
v 
’Haberlandt, Ber. D. Bot. Ges., 1900, xviii. Nemec’s paper 
appeared simultaneously in the same number. Both authors 
have published numerous later papers. I have not thought 
it necessary to give references for the many facts for which I 
am indebted to them. 
2 That is cells containing falling starch. 
