600 
NATURE 
- [April 27. 1882 . 
to be due to the weight of the end of the shoot; we 
ascribed it to epinasty, because on horizontal and inclined 
branches the hooked tip does not necessarily point verti- 
cally downwards, but is often: horizontal, or points upwards 
(Power of Movement, p. 272). Wiesner on the other 
hand states that all the ends of shoots observed by him 
at the season when the Ampelopsis is growing vigorously 
pointed downwards ; this fact would gain in value if it | 
were stated that the observations included horizontal 
branches. On the same page he remarks that the down- 
ward directed shoot often goes far beyond the vertical, 
and this, as well as the vertical position seen in other 
cases, is explained by “Zugwachsthum” (7.2. “growth 
produced by strain’’), due to the weight of the shoot. 
How this can possibly be the case is not explained. 
Diaheliotropism.—The power possessed by many leaves 
of placing themselves at right angles to the direction 
of incident light was called 7yansversal Heliotropismus 
by A. B. Frank ; we have called it diaheliotropism, partly 
for the sake of convenience and of uniformity in nomen- 
elature, and partly because our view does not exactly 
coincide with Frank’s, Wiesner asserts positively that 
diaheliotropism does not exist, and that all the phenomena 
can be explained as the result of the balance of ordinary 
forces, positive and negative, heliotropism, positive and 
negative, geotropism, epinasty, hyponasty, weight, &c. 
The main features of Wiesner’s explanation are as 
follows :—We are supposed to have a vertical bud whose 
leaves are bending epinastically down towards the hori- 
zontal position, which they ultimately assume when the 
light comes vertically from above. In this stage of growth 
before the position of the leaves is determined by that of 
the light, Wiesner speaks of the possibility of a balance 
being struck between epinasty and apogeotropism. But 
Frank (Lot. Zeitung, 1873, p. 22) has long ago shown the 
impossibility of a balance being struck between a con- 
stant force like epinasty, and a force (and this would 
apply to apogeotropism) which varies with the position of 
the organ with regard to the horizon. 
To continue Wiesner’s explanation: when the plant is 
exposed to a zenith illumination, the leaves bend down- 
wards owing to apheliotropism, and if truly apheliotropic, 
would continue to bend till they pointed vertically down- 
wards. But Wiesner believes that the light, besides 
ca ising apheliotropic movement in the leaves, has the 
power of checking their apogeotropism. If therefore the 
leaves in moving downwards go beyond the horizontal 
position, they become obliquely illuminated, and accord- 
ingly the light being weaker, the inhibition of apogeo- 
tropism is lessened, and the leaves riseup. What occurs 
if the leaves, in this upward movement, go beyond the 
horizontal is not explained, for in this case also the apo- 
geotropism would be diminished. To explain this it 
would be necessary to make more assumptions as to the 
variations in apheliotropism due to the varying obliquity of 
light, and the variations in apogeotropism due to varying 
positions of the leaves with regard to the horizon ; and 
no such assumptions are made by Wiesner. The faulti- 
ness of Wiesner’s explanation is made clear by the follow- 
ing example. Let us assume that Wiesner’s explanation 
holds good for zenith illumination, and suppose that a 
seedling dicotyledonous plant underthese conditions is sud- 
denly subjected to oblique light. Then both that cotyledon 
which is on the illuminated side, and the one on the shaded 
side will be obliquely illuminated, therefore, according to 
Wiesner, the apogeotropism of both cotyledons will be 
increased, and both should rise up; but what really 
happens is that one falls and the otherrises. This seems 
inexplicable, unless we suppose that the apheliotropism of 
the leaves differs according as the light falls, as shown 
by arrow No. 1 or No, 2 in Fig. 2. 
L a ¢ 
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Fic. 2.—Diagram representing a seedling dicotyledon exposed to oblique 
light. , the hypocotyl, or stalk; c,c, the cotyledons; the arrows 
represent the direction of the light. 
Wiesner’s explanations are also shown to be untenable, 
at least in some cases, by my observations “On the 
Power possessed by Leaves of placing themselves at Right 
Angles to the Direction of Incident Light ” (/ousnal of 
Linn. Soc., vol. xviii., 1881). I have there shown that 
certain plants are able to place their leaves at right angles 
to incident light when removed from the action of apo- 
geotropism. These experiments are not discussed by 
Wiesner, and would seem to be inexplicable from his 
point of view. When engaged in this work on diahelio- 
tropism I was struck with the impropriety of considering 
as heliotropic all movements towards a source of light, or 
all movements away from it as negatively heliotropic. 
Thus the leaves of Ranunculus ficaria move either towards 
or from the light (independently of gravitation), according 
as either of these movements is required to place the 
leaves at right angles to the direction of the incident light. 
And it is obviously impossible to call the leaves both 
negatively avd positively heliotropic.. It is far more 
rational to call them diaheliotropic ; and there is no more 
objection to the use of this term than there is to the terms 
heliotropism or apheliotropism ; all such terms are con- , 
fessions of ignorance, and none of them exclude further 
research into the phenomena to which they are applied. 
Diageotropisi.—As a diaheliotropic organ is one which 
possesses the power of placing itself at right angles to the 
direction of incident light, so a diageotropic organ is one 
which possesses the power of growing at right angles to 
the line of gravitation. Thus certain underground stems 
possess the power of growing horizontally beneath the 
surface, instead of vertically upwards, like most stems. 
In Wiesner’s few remarks on this subject, he completely 
ignores Elfving’s remarkable paper on horizontally-grow- 
ing rhizomes, which we especially referred to, and which 
is by far the most striking evidence which we possess in 
favour of the existence of diageotropism. 
Hydrotropism.—Roots have the power of bending 
towards a wet surface, and we have shown, that when 
the tip of the root is covered with a layer of grease, the 
root does not usually bend towards the wet surface, from 
which we inferred that the sensitiveness to moisture re- 
sides in the tip. Wiesner believes that the treatment to 
which the tip of the root is subjected, lessens the power 
