TRANSACTIONS OF SECTION K. 77% 
Centrifugal Force. 
Jost! objects that plants on a centrifugal machine do not behave as the theory 
would lead us to expect. Thus he found that certain roots and seedlings showed 
geotropic curvature, although the statoplasts were scattered through the cell, not 
spread out on the cell-walls furthest from the axis of rotation. Miss Pertz? and 
I have repeated some of Jost’s experiments, and have come to an opposite con- 
clusion. We find that Setaria does not curve with a centrifugal force of less 
than 0:02 g., and this is about the limit for visible displacement of the starch- 
grains. As the centrifugal force increases up to 0:04 we get slight amounts of 
curvature and slight amounts of starch displacement. The two phenomena 
cannot be accurately compared, but so much is clear: that the result of Knight's 
experiment is not destructive of the statolith theory, but, on the contrary, is 
roughly in harmony with it. 
The result of an intermittent stimulus may seem to some a difficulty. 
Jost* produced geotropic curvature by placing seedlings in the horizontal and 
vertical positions for alternate periods of 34 minutes. With alternate periods of 
50” horizontal and 2’ 30” vertical he sometimes failed to get a geotropic curve, 
and exposures if less than 50” always failed. It is commonly said that 
15-25 minutes are needed for the starch to fall on to horizontal cell-walls, and it 
may seem, therefore, that in these experiments neither 3} minutes (nor, a fortior?, 
50”) could produce a change of position in the statoliths, and that therefore the 
experiment is destructive to the theory. But this would be a wrong conclusion, 
for, according to my experience, the falling time of starch is often less than 
15 minutes; and even if this were not so there would be no difficulty in under- 
standing the above experiments, for, as Jost allows (loc, cit.), and as Némec (02) 
has also pointed out, the statoplasts may stimulate the cell without the occurrence 
of any visible displacement ; for if the statoplasts do not fall over and spread out 
on the horizontal walls there must be a column or heap of starch-grains, whose 
height equals the width of the cell, resting on the lateral wall of the cell instead 
of, as in the normal position, a shallower layer pressing on the basal wall. Here 
we have plain conditions of differentiation between the vertical and horizontal 
positions. 
The same considerations apply to the whole question of what is known as 
the geotropic presentation time*—.e., the minimal period of horizontality needed 
to induce a geotropic curvature. It has been said that the presentation time 
corresponds with the time needed forthe statoliths to fall on to the horizontal walls 
of the sensitive cells. It seems to me that we hardly have knowledge enough 
to be certain of this coincidence, and since, as above pointed out, the statoliths 
may begin to stimulate before they are visibly displaced, the question is not one of 
much interest or deserving of special inquiry. 
Theoretical. 
Elfving’s® well-known experiment with grass haulms shows that (in this in- 
stance) the action of the klinostat depends, not on the prevention of all gravi- 
perception, but on the equal distribution of stimulus.6 But other plants react 
differently—that is to say, they do not exhibit increased rectilinear growth on the 
1 Jost (02). 
? Darwin and Pertz (04). By an oversight we omitted to give a reference to 
Némec’s (02, p. 347) interesting reply to Jost’s criticism. 
* Jost (02), p. 175. See also Czapek (98), p. 206; and Noll (00), p. 462. 
4 Czapek (98), p. 183. 
° Elfving (84) proved that the pulvini of grass haulms increase in length when 
kept in slow rotation on a klinostat. 
® My experiments on the germination of Cucurbita demonstrate the same point 
(Darwin and Acton, 94). Czapek (02, p 469) shows that the homogentisin 
reaction occurs on the klinostat, 
“ . 
aD2 
