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April 20, 1882] 
“NATORE 
581 
have a mode of growth which may almost be said to be 
halfway between circumnutation and the revolving nuta- 
tion of climbing plants. The scattered distribution of 
climbing plants throughout the vegetable kingdom proves 
the wide distribution of a form of growth from which re- 
volving nutation is developed—and this form of growth is 
circumnutation. This widespread form of circumnutation 
cannot therefore be dismissed as Wiesner has done, since 
such a treatment of it is quite beside the question. 
‘General Mechanism of Movement.—tin “ The Power of 
Movement” we have spoken of the movements of plants 
as being due to difference in turgescence and in ductility 
on opposite sides of the moving organ. In it we have 
pointed out that it is more correct to look at the difference 
in turgescence than to difference of growth as the primary 
cause. In this statement “ growth’? was meant to mean 
alteration in size due to intercalation of solid particles. 
If a turgescent stem is allowed to bend heliotropically 
and is then placed in a salt solution strong enough to 
destroy its turgescence the heliotropic curvature is in 
large part destroyed ; showing that the curvature at any 
given moment is largely due to differences in turgescence. 
Wiesner (p. 33) made the following experiment: he 
took five seedlings and found that when placed in salt 
solution (10 per cent.) they became shorter on the average 
by 1'9mm. ; five other similar seedlings he allowed to 
grow for 43 hours, during which time they increased in 
length by 6:2 mm. (average), Wiesner argues that if 
the whole increase in length during 4} hours were 
due to turgescence, then the shortening caused by 
salt solution ought to = 6'2 mm. + the original 1°9 mm. 
which was shown to be in a turgescent state by the 
former experiment; this, however, was not the case. 
It would be almost as reasonable to measure the length 
of an epicotyl in a dormant seed, and to expect that 
after germinating and growing for a day or two the 
young plant should collapse in salt solution to the size of 
the rudiment which existed in the seed. I imagine that 
it is a generally received opinion, and one which does not 
require Wiesner’s experimental demonstration, that in- 
crease of length by turgescence and the intercalation of 
solid matter proceed simultaneously. 
The question, however, need not be considered in 
further detail, for on this point there is practically no 
difference between Wiesner’s and our own view; he says 
(p. 35) that “growth is from the first a combination of 
several processes occurring simultaneously, of which how- 
ever Turgor is at first the governing one (vorherrscht).” 
This view is the same as that of Sachs (‘‘ Lehrbuch,” 
Eng. Tr., p. 712), who describes the interaction of tur- 
gescence and intercalation. 
Hreliotropism.—Wiesner’s criticism on the new matter 
contained in our book with regard to Heliotropism is 
prefaced by a discussion of some length on the nature, 
&c., of the phenomena of Heliotropism. There is only 
one point in this discussion which I wish at present to 
call attention to. Wiesner holds to De Candolle’s ex- 
planation of heliotropism, namely, the purely mechanical 
view that the convex side grows more quickly, simply 
because it is in shade. As this view does not account for 
apheliotropism since at least some apheliotropic organs 
also grow more quickly in darkness than in light, Wiesner 
assumes (p. 55) the existence, in the fibro-vascular bundle, 
of negatively heliotropic elements, whose growth is as- 
sumed to be favoured by light. It is a pity that this theory 
has not been at least partly tested by comparing the rate 
of growth of unicellular apheliotropic organs in light and 
darkness. : 
Again the theory seems to require that all aphelio- 
tropic organs should be positively heliotropic in light of 
low intensity ; and this, as far as I know, is not the case 
with the roots of Szapis alba, which I have shown (“ Ar- 
beiten des Bot. Inst. Wiirzburg,” Bd. ii., Heft. 3) to be 
apheliotropic even with very dull light. But I do not lay 
great stress on this argument, as Wiesner’s theory seems 
to me to rest too much on assumptions to be at present 
capable of being discussed. 
Transmission of Heliotropic Stimulus.—In ‘The Power 
of Movement” (Chap. ix.) facts were given that seemed 
to us to show that one part of an organ may bend helio- 
tropically owing to the illumination of another part, 
and that, therefore, a heliotropic stimulus is transmittable 
from one part of an organ to another. The experiment 
which led to this conclusion is as follows:—A number of 
small glass caps were prepared, some of which were left 
transparent, the others were painted black. These caps 
were then slipped over the tips of seedlings of Phalaris 
(canary grass), which were thus prevented from bending 
towards the light, although they did so when the unpainted 
glass caps were used. The same experiment was made 
with other seedlings—cabbage among the number. From 
these results it was inferred that the illumination of the 
upper part of the plant was necessary to make the lower 
part bend towards the light ; or, in other words, that the 
heliotropic curvature of the lower part depends on some 
influence transmitted from the upper and illuminated part. 
In the case of cabbage seedlings we found that if the 
lower part is darkened, while the upper is illuminated, no 
heliotropic curvature of the lower part takes place. Thus 
we believed that the lower part is in some degree helio- 
tropic, independently of the illumination, of the upper 
part, although the latter is the most important factor in 
the illumination of the plant. This seems to have been 
misunderstood by Wiesner, whose criticism is largely 
directed against what he believes to be our view, namely, 
that a heliotropic stimulus can be transmitted to a part of 
an organ which is not itself heliotropic. This misunder- 
standing on the part of Wiesner is no doubt due to a want 
of clearness in what we have said on the subject. 
Wiesner speaks with great positiveness on this subject, 
and asserts that he has shown to demonstration that our 
experiments do not justify our conclusions. He believes 
that the bending of the lower part of the plants observed 
in our experiments is due to what he calls “Zugwachsthum” 
(p. 72), that is to say, the effect of the weight of the upper 
part of the plant acting on the growth of the lower part. 
We have discussed the question whether the curvature of 
the lower part of the cotyledons of Phalaris can be due to 
weight, and have shown conclusively that it cannot be so 
(‘Power of Movement,” p. 469), but Wiesner makes no 
mention of this experiment. Pots of Phalaris seedlings 
were placed on their sides so that the cotyledons were 
horizontal and were at right angles to the direction of the 
incident light from a bright lamp. Under these circum- 
stances they became bent close to their bases, nearly the 
whole cotyledon being thus directed towards the light. 
