FEBRUARY 15, 1901.] 
roots bending positively when a pin or a 
thin wooden rod was brought against the 
root 2 to 4 mm. back from the apex. Wies- 
ner and others have shown Darwin to be 
mistaken, and the author of the paper here 
abstracted has repeated Sachs’ tests many 
times without convincing results. 
That roots are, however, responsive to 
pressure on the elongating zone can be 
shown by two kinds of experiments. Seed- 
lings of buckwheat or radish are placed 
upright with their roots immersed in water, 
and a loop of very thin rice paper attached 
to a light pendulum is made to pull lightly 
on the elongating zone. Not more than 
half the roots bend, but all that do curve 
become concave on the side pressed by the 
paper. A better method is employed when 
gravitation is neutralized by revolving the 
seedlings in a vertical plane by the use of 
a klinostat. In this experiment the seed- 
lings are supported in a damp chamber 
while their elongating zone rests lightly on 
a fixed glass rod. In sensitive roots, the 
tip of the root curves partially around the 
glass rod as growth goes on. These ex- 
periments show that some species respond 
and some do not respond to pressure. As 
far as the study has been carried, roots 
which are rheotropic are also thigmotropic. 
This agreement is strong evidence for the 
view that rheotropism is really thigmotro- 
pism. Neither rheotropism nor thigmo- 
tropism would seem to be of biological im- 
port to the plant. The response may be of 
the same class of phenomena as shown by 
tendrils when in contact with a solid object. 
The Effect of Mechanical Shock on Longitudi- 
nal Growth of Plant Organs: Dr. Jas. B. 
Pottock, University of Michigan. 
The plant organs used were hyphe of 
Phycomyces, hypocotyls of Brassica, Raph- 
anus, Helianthus, Lupinus, and Cucurbita, the 
epicotyl of Phaseolus, and the leaf sheath 
and first leaves of Avena and Triticum. 
SCIENCE 
251 
Single shocks were given by pressure 
(Phycomyces) as by forcibly bending from 
side to side. 
In Phycomyces there was a retardation 
after pressure, then a recovery in 5-30 
minutes, usually in about 10 minutes, and 
the growth was then sometimes faster and 
sometimes slower than at first. 
On bending, the larger plants first elon- 
gated considerably, were then retarded for 
a short time and recovered in 20-50 min- 
utes, so they grew at a fairly constant rate, 
this rate being sometimes greater, some- 
times less than at first. 
Continuous shock was produced by sev- 
eral pieces of apparatus worked by elec- 
tricity, water motor or clockwork, and was 
either a swaying from side to side or a jolt- 
ing upon a board hinged at the middle. 
The results were very decisive only in 
the case of Cucurbita, and showed a de- 
cided acceleration, due perhaps to the 
swaying from side to side. With all the 
other plants used the results were quite 
variable, but, taken as a whole, give evi- 
dence of acceleration as the result of not 
too vigorous swaying from side to side. 
The Limits of Variation in Plants: Dr. Jonn 
W. HarsHperaer, University of Penn- 
sylvania. 
The study of the limits of variations in 
plants was undertaken as in part a contri- 
bution to the problem of species. It was 
stated, as a well-known fact, that more 
plants are produced than can survive, 
necessitating the destruction of many, and 
the survival of those that have fitted them- 
selves by certain aptitudes to do so. As in 
part an answer to the evolutionary diffi- 
culty of small or initial variations, a careful 
statistical inquiry was made by comparative 
measurements of various plant parts. It 
was found, that the size and shape of leaves, 
the weight and size of fruits varied by math- 
ematically ascertainable quantities. These 
