598 
NATURE 
[| April 27, 1882 
intensity of light and amount of curvature; and an ex- 
planation which requires so large an assumption cannot 
be considered a simple one. I shall therefore at present 
neglect the existence of the optimum, and consider the 
heliotropic curvatures produced by degrees of light all 
below the optimum. Here we see plainly enough that the 
intensity of the light is by no means proportional to the 
curvature. 
Now Wiesner’s explanation is that “the difference of 
illumination on the lighted and shaded sides, which 
causes the unequal growth leading to heliotropism is not 
proportional to the intensity of the light.’’ This state- 
ment is in opposition to the laws of Optics. IfJ is the 
intensity of light per unit of surface, illuminating the light 
side, then the amount of light absorbed in passing through 
the plant isacertain proportion of J (call it AJ), therefore 
the illumination on the darker side is J—-AJ=J(1—A); 
and the difference between the illuminations on the two 
sides is AJ, that is to say, the difference is proportional 
to the intensity of the light. [The proportion between the 
J 
jG=h) TSA 
Wiesner -might connect the heliotropic facts with his 
observations on the amounts of longitudinal growth under 
different intensities of light (Monograph, Part ii. p. 12 
and following pages) ; but this is only pushing the diffi- 
culty one step further back. The fact remains that the 
way in which growth is checked by light does not re- 
semble the effect of light on photographic paper, but is 
more like the effect of light on sensitive animal tissues." 
Sachs and his pupil Miller Thurgau made known long 
ago the important phenomena of ‘ Nachwirkung,” or 
after-effect. If a plant is laterally illuminated for a certain 
time and then placed in the dark, the curvature will be 
continued in the same direction as though light was still 
actins on it. Or if, instead of being darkened. it is simply 
turned round, so that the direction of the light is reversed, 
the plant will go on curving in the direction in which it 
has begun to do so, in spite of the light now illuminating 
the opposite side. This phenomena we compared with 
the after effect of light on the retina. But Wiesner will 
not allow this comparison, chiefly because he finds an 
analogy between the after effect of light on plants and the 
photo-cheinical induction of Bunsen and Roscce (Wies- 
ners Monograph, i. p. 66). But we may just as well say 
that the effect of light on the ve¢7za is comparable to photo- 
chemical induction. We should then be able not only to 
say that light may in some respects act in a similar way 
on plants and animals, but (thanks to Wiesner) we should 
be able to form a theory as to why this is so. 
In the “ Power of Movement” we recorded our belief 
that plants are sensitive tothe successive contrast of light 
and darkness, and this seems to us another point of re- 
semblance between the sensitiveness of plants and animals. 
Thus, plants which have been exposed to the daylight 
did not become so quickly heliotropic as others, which 
had been kept in darkness. Again, consistently with 
this view, we attributed the striking effects of inter- 
mittent light to the increased sensitiveness of plants kept 
in the dark. 
With respect to the effect of intermittent light, Wiesner 
states that the heliotropism produced by intermittent 
* Of course no one denies that stimulation ultimately depends on the 
physical properties cf the sensitive tissue. 
two intensities is , which is constant. ] 
light is not greater than that due to constant illumination. — 
The fact that so much heliotropic curvature is produced, — 
is supposed to be due to the after-effect of the periods of 
illumination being continued through the periods of dark- — 
ness, and thus producing an effect equivalent to con- 
tinuous illumination. This naturally occurred to us, and 
it may possibly be the correct explanation. 
Our statement that plants are more sensitive to light 
if they have been kept in the dark, Wiesner considers to © 
be well known to be true, in the case at least of etiolated 
plants. He explains it by the increased capability for — 
growth which is produced by darkness. On the other 
hand, he quotes an experiment from his ‘‘ Monograph”’ 
to show that a plant subjected to equal illumination on all 
sides is rendered more sensitive heliotropically than a 
plant grown in the dark. This fact he considers as quite 
destructive of our view on the effect of contrast. He 
explains the result as follows. 
The turgescence of the cells of an organ offer a certain 
resistance to its curvature, and therefore anything that 
slightly diminishes the turgescence increases the power of 
bending heliotropically ; and as bright illumination di- - 
minishes turgescence, it may be expected to increase the 
heliotropic sensitiveness. This is such a contradiction to 
his explanation of our results and to other statements of his, 
that I should be inclined to think that I have misunderstood 
him. But we have on the same page (p. 83) our obser- 
vations explained as the result of the increased ‘ capacity 
for growth’’ caused by darkness, while his own observa- 
tions are explained by diminished turgescence, which must 
imply diminished capability for growth as its chief result. 
If Wiesner explains his own observations correctly, he 
cannot explain ours, and therefore his criticism is deprived 
of all force.* 
Geotropism—Sensitiveness of Root-tips to Gravitation. 
—lIf the extreme tip of a root (1 to 1°5 mm.) be cut off, 
the root will continue to grow vigorously, but it ceases, or 
almost ceases, to be geotropic.? We concluded that when 
a root is placed horizontally, the force of gravity acts on 
the tip, and a stimulus is transmitted to the growing part of 
the root, causing it to bend down. And that accordingly 
when the root-tip is destroyed the root is incapable of 
receiving the stimulus of gravitation. Wiesner differs 
entirely from this point of view, and believes that the 
diminished geotropism of the pointless roots is simply due 
to their diminished activity of growth due to the injury. 
I have elsewhere treated this question experimentally, 
| and have been able to show that if the tip is not cut off 
roots may be very severely injured in other ways, so that 
the growth is greatly retarded, and that they are never- 
theless capable of geotropic curvature. I have thus shown 
that Wiesner’s explanation of the facts cannot be the 
right one, since mere retardation of growth is not suffi- 
cient to account for loss of geotropism. 
Sensitiveness of Root-tifs.—In “The Power of Move- 
ments” we showed that the tips of roots are stimulated 
| in a peculiar manner by the continued contact of small 
squares of cardboard cemented to one side of the ex- 
treme tip of the root. It was found that the root curved 
1 Tt should also be observed that Wiesner’s experiments were made with 
etiolated plants, which was not the case with ours. It appears (Monograph, 
p. 8) that in /. Fadsa and Soja hispida no increase of helitropic sensibility 
was produced by illumination. 
= “Power of Movement,” Chap. x. 
point is due to Ciesielski. 
The original experiment on this 
