770 REPORT—1904. 
amputation. On the other hand, Némec! states that geotropism persists, if the root- 
tip is cut half through by two opposite incisions in different planes, so that the 
whole of the tissues are divided, and yet the tip is not amputated. Thus four out 
of five bean roots treated in this way showed distinct geotropism in 5} hours. 
This seems to me a striking result, as showing that the shock of the operation is 
not exclusively the decisive element. Némec has, moreover, shown that if geo- 
tropic curvature has begun on a normal root, a wound interferes with the amount 
of after-eflect, and that the precise nature of the wound is not decisive, and this, 
as far as it goes, confirms the assumption that two half-cuts would produce as 
much shock as actual amputation. 
Czapek* finds that splitting a bean-root longitudinally has the same effect as 
decapitation. This would mean that decapitation produces its results by shock 
only, since in a split root there is no removal of the tip. I think I was the first to 
make use of the splitting of roots in this connection. I wished to show® the 
incorrectness of Wiesner's view—viz., that amputation prevents geotropism by 
checking growth. In my experiments the split roots were greatly checked in 
growth, but curved geotropically, behaving in this respect quite differently from 
amputated specimens. 
Another striking bit of evidence on Czapek’s side of the question‘ is the fact 
that Lupin roots from which ‘3mm. of the tip has been removed, and which, 
therefore, contain no statoliths,’ show the remarkable homogentisin reaction which 
he has convincingly proved to be a symptom of graviperception. Ozapek adds 
that the same is true of roots from which 1 mm. has been removed. It seems to 
me that Némec’s reply to this® is of value. He finds that the root-cap in Lupin 
is variable in length, but always longer than } mm.; therefore, in the roots from 
which } mm. only was removed there should have been some statocyte tissue 
remaining. Even after the removal of 1 mm. the root can, according to Némec, 
rapidly form statocytes, since the section is in the neighbourhood of the 
calyptrogen.° 
Némec suggests it to be conceivable that differences of pressure in Ozapek’s 
sense may give rise to the homogentisin reaction, while the true act of gravi- 
perception is confined to the statoplasts. This is no doubt possible, but I 
confess that, if the homogentisin reaction can occur in root-tips which have no 
statoliths I should consider it a strong argument in favour of the view that 
pressure-difference in Czapek’s sense supplies the machinery of perception in roots. 
Czapek also claims that his experiments with bent-glass tubes (Czapek, 95) prove 
the graviperceptive region of the root not to be confined to the region of stato- 
plasts, since if the root-cap alone isin the vertical branch of the tube, geotropic 
curvature is not excluded. Némec (04) has attempted a rejoinder to this objec- 
tion ; with what success readers must judge for themselves. 
It will be seen that, in my opinion, the balance of evidence is not fatal to the 
statolith theory. Czapek, who treats the question in a broad and liberal spirit, 
is by no means inclined to deny that statoliths haye a share in graviperception ; 
all he claims to prove is that the statoplasts do not supply the whole of the 
mechanism. It is not easy for an upholder of the theory to allow this much in 
the present stage of the controversy. The best way of testing the theory is by 
comparing the distribution of geotropism with that of statoliths; and if we are to 
allow, in all cases which are opposed to the statolith theory, that the stimulus 
depends on pressure differences in Czapek’s sense, we deprive ourselves of the 
best means of proving the truth or falsehood of our theory. Those who uphold 
the theory must have the courage of their opinions and finally trust to the facts 
of distribution, But further knowledge is necessary before such a judgment can 
fairly be made. ‘ 
y ee (Ol, p. 19). ? Czapek (98, p. 202), and (02, p. 118). 
3 F. Darwin (82). 
* Czapek (02, p. 468). 5 Némec (04, p. 53). 
° He adds that the calyptrogen may in this way have an indirect importance, 
and Firtsch’s belief that this tissue was the essential seat of graviperception may 
be accounted for. 
