260 
PROFESSOR MACATRE ON THE DIRECTION ASSUMED BY PLANTS. 
passed under the diaphragm and had reached the illuminated part of the vessel, 
it grew in an erect position, became green, and began to develope leaves. 
This experiment, very often repeated with peas, French beans, &c., always gave 
similar results. Never did any attraction of the whole plant towards the light mani- 
fest itself ; and however long a stem it might be necessary to produce in order to 
reach the diaphragm, that was always the means to which the plant had recourse. 
In order to vary the mode of experiment, I placed on a cork float in the obscure 
part, a plant of French beans already developed, green and strong, and the roots of 
which were immersed in the water. The green stem took little or no ulterior deve- 
lopment ; but from the neck of the root there grew out another stem, white and 
etiolated, that spread itself along the water to reach the diaphragm and the light 
portion of the vessel ; there it grew erect and gave forth its leaves. The float had 
not changed its place, and, as before, had only been bent down by the weight. The 
same experiments repeated with green and vigorous peas, gave precisely the same 
results. 
Germinated seeds of mustard placed on very light floats in a tumbler half-full of 
water and surrounded with black paper, so as to admit only a luminous ray, were 
put in the dark portion, but very near the aperture. One of the plants grew and 
sprouted a stem that went all round the tumbler to come and spread its leaves in 
that part of the vessel where the luminous aperture was, and once there, the plant 
did not pass beyond that point. It grew erect in that part, though the light was too 
weak to render it entirely green. No motion was perceptible in the float ; and it is 
very remarkable that, being close to the luminous aperture, the plant in search of 
the light had taken this long circuit rather than communicate to the float the 
slight motion that would have placed it in possession of it. I venture to conclude 
from these experiments, that the direction of plants towards the light is not the 
result of an attraction, properly so called, which, similar to the physical attractions, 
could carry over the attracted body towards the agent. 
The ingenious supposition of Decandolle of a mechanical bending, due to a greater 
solidification of the fibres on the light side than on the other, appears to me not to 
be more applicable to the direction of stems towards the light, than to the curling 
of tendrils ; for, by the arrangement of the apparatus, the lightest side was not always 
the same, and the stems advanced straight towards the light without incurvation or 
bending. 
There remains to examine the hypothesis of Dutrochet, who admits that there 
exists in the stems and branches of plants a system of cells progressively increasing 
in size from the centre to the circumference in the central parts of the stem, and from 
the circumference inwards in the cortical portion. He supposes that these cells are 
gorged with sap by the influence of endosmose, and that this endosmose tends to give 
a bending in contrary directions to the two systems, so that the stem is inflected in 
the direction that predominates. He admits too the existence of a fibrous system 
