- remark. 
580 
NATORE = 2 
9 
[Apri 20, 1882 . 
ture). He believes that first one and then another of 
these forces gets the upper hand, so that the tip of the 
root moves backwards and forwards. 
To this explanation there are several objections. (1) If 
the root is pointing vertically upwards, as in some of 
Wiesner’s experiments (p. 174), any tendency to spon- 
taneous curvature in the root will assist geotropism, so 
that any circumnutation that may occur is inexplicable as 
the result of antagonism of the above-named forces. (2) 
Wiesner states (pp. 169, 174) that the movements did 
not take place in one plane: his explanation does not 
account for this fact. (3) He states (p. 172) that the 
“so-called” circumnutation takes place in the part of 
the root which grows most quick. But ‘‘ Sachs’ curva- 
ture,” which he assumes to be one element in the “so- 
called” circumnutation, takes place at the base of the 
root. It is true that it may be said in favour of Wiesner’s 
view that the root may be carried out of the vertical by 
Sachs’ curvature ; and if this were the case geotropism 
would bring it back to the vertical, and thus the direction 
of the circumnutation would correspond more or less with 
the plane of Sachs’ curvature. Lut this would not account 
for movement in any other piane. 
Circumnutation of Stems.—Wiesner concludes that 
there exist stems of plants which certainly do not circum- 
nutate at all. This statement he founds (pp. 176, 177), 
however, on observations on plants whose line of growth 
is not a straight line, but is broken by lateral oscillations 
in various directions. The lateral movements being small 
and irregular, however, are held not to constitute circum- 
nutation. In one case the growth of the grass seedlings 
under observation was at first accompanied by the above- 
mentioned very minute and irregular curvatures, but 
afterwards seems to have circumnutated in our sense of 
the term. I shall return again to these cases. 
In the case of stems which show the S-shaped curve of 
“undulating nutation,” Wiesner observed the tip (Faba, 
p. 178) move backwards and forwards in the plane of cur- 
vature ; this he explains as the summation of the apogeo- 
tropic curvature of the lower part of the stem with the 
nutation (ze. curvature) of the upper part. 
Unless I misunderstand Wiesner in this point, it seems 
to me his explanation does not meet the facts; for I fail 
to see how summation of two curvatures can produce 
anything, except a variation in the fapidity of the curva- 
ture. I cannot see how it accounts for any movement in 
the opposite direction. 
Nor again does Wiesner give any explanation of the 
movements which he observed in the plane at right 
angles to the nutation-plane. It should, however, be 
mentioned that in the epicotyl of the bean; Wiesner 
observed perfect straight growth after the undulatory 
nutation had ceased (p. 178). 
Wiesner’s observations on heliotropism (p. 182) in con- 
nection with circumnutation do not call for any special 
He seems not to have taken the precaution to 
expose the plants experimented on to a dw// light, and 
the plants consequently curved in nearly or quite straight | 
ee 
importance. 
The movements of the flower-head of the daisy (p. 183) | 
lines towards the light, as occurred in our experiments. 
Wiesner puts down to the effects of the weight of the 
flower-head. Healso assumes that the circumnutation 
which he observed in a flowering spike of a Plantago is 
due to the irregular disposition of the florets on the 
inflorescence. 2 * 
Circumnutation of LeavesiA—On this point Wiesner’s— 
views are briefly :—(1) Some leaves grow in absolutely 
straight lines without circumnutating. (2) He confirms 
the facts observed by us, namely, that the tip of the leaf 
does describe the complicated figures described by us as 
circumnutation, but he interprets the facts differently. 
He believes that the complicated forces acting on the 
leaves, viz. epinasty, apogeotropism, apheliotropism, influ- 
ence of weight, &c., working in antagonism to one 
another, and alternately getting the upper hand, produce 
the movements in question. 
This argument is one of the most important which 
Wiesner makes use of, and a careful consideration such 
as it deserves would require further experiment and ob- 
servation. It is obviously difficult to distinguish between 
circumnutation modified by the contending forces, and 
the same contending forces acting on an organ without 
circumnutation. One feature in our observations is the 
almost constant presence of movements in a horizontal 
plane, movements therefore which cannot be produced 
by any of the contending forces above described. Against 
the numerous cases in which sideway movements occur, 
it may be mentioned that, according to Wiesner, cases 
occur in which no lateral movements can be observed 
(p. 192). 
This will perhaps be a convenient place to discuss the 
minute irregular disturbances which Wiesner usually 
found to exist even where the organ did not properly cir- 
cumnutate. Let us for a moment compare circumnutation 
with variability. Modifications of organs are brought about 
by the summation of small variations in certain directions, 
and thus we rightly consider variability as the necessary 
groundwork for modification. But we find some animals, 
é.g. the common goose, which may be almost said not to 
vary, yet we do not, on the strength of this fact, assert _ 
variability is not necessary for modification. No two 
organs are mathematically similar, yet we cannot draw a 
distinction between minute irregular deviations from the 
normal, and such plain deviations as are called variations. 
In the same way it may be said that no fast and firm line 
can be drawn between circumnutation and the minute 
irregular disturbances of Wiesner. We have shown that 
a true circumnutating movement (as in Brassica) is made 
up of very small irregular jerks, and this may be given as 
another reason for believing that the two kinds of move- 
ment are only extreme forms of the same phenomenon. 
In summing up what he has to say on the subject 
of circumnutation, Wiesner says (p. 202) that the move- 
ments described by us as circumnutation are either 
disturbances of growth or they are produced by com- 
binations of antagonising forces, or they are identi- 
cal with the revolving nutation of climbing plants. 
From Wiesner’s brief manner of dismissing the last 
mentioned class, it might be supposed that it has little or 
no bearing on the question. But this is far from being 
the case; it is precisely this class to which we attach 
There can be no doubt that revolving nuta- 
tion of climbing plants is a development or exaggeration 
of circumnutation. In the stolons of the strawberry we 
It is curious that Wiesner (p. 126) recommends the use of glass fibrés 
affixed to a stem for observing circumnutation, while on p. 187 he suspects 
that circumnutation of leaves is disturbed by attached glass fibre. 
