PROFESSOR MACAIRE ON THE DIRECTION ASSUMED BY PLANTS. 
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placed in the central cellular organization, and supposes that its fibres are also de- 
creasing in size and are inflected by being filled with gas or oxygen endosmose. 
The incurvation of this tissue always takes place inwards, and that of the cellular 
system outwards. According to Dutrochet, the influence of light has a tendency to 
diminish the filling up of the cells and impair their power of incurvation, and, on the 
contrary, to increase the production of oxygen gas that creates a bending in the op- 
posite direction in the fibrous tissue. He has grounded this theory on some curious 
experiments. For instance, by cutting in a longitudinal direction a stem of Medicago 
sativa bent towards the light, he saw that the part of the stem which had been in 
the light increased its curvature, while the other, on the contrary, became first 
straight and then bent in an opposite direetion. There was then, according to him, 
an antagonism between the two slips, and this fact was sufficient to overthrow 
Decandolle’s theory, in which, as the exterior slip is supposed to be the only active 
one, its incurvation ought to have been preserved or even increased in the primitive 
direction, I have repeated the same experiment with the same result on many stems 
or footstalks, among others on the footstalk of the leaf of Calla cethiopica. But in 
trying to repeat the sections in all directions, 1 observed that the bending of the 
slips was always towards the bark, or outwards, and that the position of the stem 
with regard to the light had nothing at all to do with it. This phenomenon is due 
to the contraetion or the resistance to any elongation of the exterior cutiele. 
Thus, when a segment of stem or footstalk is immersed in water with its cuticle 
entire, the liquid, penetrating into the cells, fills the cellular tissue, renders it turgid, 
and there results an elongation of the central parts ; but as the fibres of the cutiele 
cannot follow this elongation and therefore resist it, the segment of stem or stalk is 
infleeted towards the resisting side, or the cuticle, and the incurvation becomes 
manifest outwards. If, leaving the central cellular system entire, I only removed the 
cuticle, the immersion of the slip in water, though it brought on the turgid state, did 
not create incurvation, which yet ought to have appeared if it were, as Dutrochet 
supposed, a necessary result of the cells decreasing in size. The same consequence 
followed when the segment was immersed in water after the cuticle had been slit 
in two or three places, taking care that the ineision did not go beyond it ; the im- 
bibition took place, the cellular system was swollen and elongated, but the stem 
remained straight because the cuticle no longer presented any resistance. 
The cuticle alone put into water curls by the shortening of the fibres pretty nearly 
like a wet rope, so that, far from lengthening it, the liquid has a tendency to 
shorten it. 
Besides, Dutrochet’s hypothesis that light has a tendency to diminish the endos- 
mose by the vacuum produced by the exhalation in the cells, appears not to agree 
with the supposition of the same author, by which he attributes the ascension of the 
sap to endosmose. He admits that it is through the cellular membrane of plants 
and by endosmose from cell to cell that it produces the ascensional mareh of sap, a 
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