4i 
The Localisation of Perception. 
the position BC„, the region C 2 has ceased to grow, in which case 
the curvature at C 2 must be permanent. If we imagine this 
process to continue, i.e., each fresh curvature occuring close to B 
and becoming permanent, then it is obvious that the seedling will 
curve in one direction indefinitely. Personally I am not inclined to 
believe in the occurrence of such a type of curvature in this parti¬ 
cular case, but until certain experiments are completed I cannot 
give my reasons. 
One other experiment should be mentioned on account of the 
very ingenious method employed. Piccard 1 experimented with 
germinating beans which he attached to a wire frame rapidly 
rotated by a centrifugal machine. The seedlings were so placed that 
the root-tip was on one side of the axis of rotation and the region 
of curvature was on the other side. In other words a point half¬ 
way between the supposed sensory region (the tip) and the motor 
region was in line with the axis and therefore unaffected by centri¬ 
fugal force, while the motor and sensory regions were subjected to 
centrifugal forces acting in opposite directions. Piccard obtained 
varying results, but the majority of his experiments showed an S 
curve, due to curvatures produced by both the opposite centrifugal 
forces. This would mean that both the tip and the motor region 
are gravi-sensitive. It is to be hoped that this by no means easy 
experiment may be repeated and the question finally decided. 
Heliotropism. 
The fact that in the case of heliotropism, the sense organ for 
light-perception is localised in the apex of certain seedlings is 
familiar, nevertheless a few words may be added on this subject. 
The adaptive character of this arrangement is more obvious 
perhaps in the case of heliotropism than in geotropism. It is clear 
that a germinating seedling will more easily make its way through 
superincumbent debris if the part that goes first is the perceptive 
region. 2 The analogy with animals is sufficiently obvious and need 
not be insisted on. 
The fundamental experiment first described in the Power of 
1 Pringsheim’s Jahb., XL., 1904, p. 94. 
2 We may perhaps see evidence of adaptation in the distribution 
of light-perception in time as well as in space. The young 
seedlings of Ipotnaa purpurea and ccerulea are strongly helio¬ 
tropic, whereas the stems of young plants, are practically not 
heliotropic, as proved by the fact that in circumnutating they 
curve to and from the light in times that are nearly identical. 
This symmetrical type of circumnutation is clearly advan¬ 
tageous. 
