148 ANATOMY AND PHYSIOLOGY OF 



leave out of the question tlae doubt above referred to, tbe complete un- 

 certainty whether light can cause greater amount of discharge of aqueou& 

 juices from the cells of the illuminated side of the rind of plants lying 

 under water ; but we must therefore the more distinctly advance that the 

 statements of the anatomical facts given by Dutrochet are altogether 

 erroneous, he himself having contradicted these statements by anatomical 

 facts mentioned in other paits of his writings. With regard to the rind 

 of the stem of plants striving towards light, the statement that these 

 curve inwards in water is most decidedly incorrect, of which I have con- 

 vinced myself ia a great number of plants, and in particular in the Phy- 

 tolacca cUcandra, especially cited as an example by Dutrochet, for the 

 rind of aE the plants which I investigated in reference to this question 

 curved outwards in water. It is equally untrue that the rind of roots 

 curves outwards, for in most roots just the opposite occurs. But when 

 Dutrochet, in explaining the avoidance of light by roots, ascribed the said 

 structure to their rind, he forgot that he had stated directly the reverse 

 of this structure of the rind, in explaining the direction of the root down- 

 ward. In this way he mixed the anatomical facts together, like a con- 

 juror does his cards, just as they were reqxiisite at the moment to explain 

 any movement. Dutrochet had still another subsidiary hypothesis for 

 the explanation of the movement of the stem, according to which the 

 fibrous parts of the stem, i.e., the young wood, were caused to curve out- 

 ward by absorption of oxygen. He states that as light sets free oxygen 

 in the green cells of the rind, a portion of this is conveyed to the young 

 wood, the latter then, by curving, assists the curvature commenced by 

 the rind. Disregarding the fact that the entire theory of the curvature 

 of the wood through the absorption of oxygen rests upon very uncertaia 

 experiments, two circumstances are opposed to it ; in the first place, the 

 curvature of plants is almost exclusively produced by blue light, while this 

 is completely incapable of causing evolution of oxygen from gTeen parts ; 

 in the second place, according to Payer's experiments (" Comptes rendus" 

 184:2, 26th December), the curvature takes place also in nitrogen and hy- 

 drogen. Since the pretended curvature of the young wood would be in the 

 way of the movement from the light in the tendrils of Vitis, in the shoots 

 of the "Weeping Ash, &c-, they are eliminated by the statement that in these 

 plants the young woody layer is so thin and weak that its effects are im- 

 perceptible. It is clear the author knew how to get out of a difficulty. 



While the stem and the root only exhibit movement when they 

 seek to regain the amtnral position ont of which they have been 

 removed, it is different id the leaves, for these have not only the 

 power, in a high degree, of returning to their natural position, 

 when artificially disarrangBd, but (with the exception of the stout 

 leathery or fleshy leaves) almost all thin leaves, and particularly 

 compound leaves, present different arrangements by day and by 

 night, a phenomenon to which the terms sleeping and waking 

 have been applied. As in the stem the normal position is the 

 perpendicular, with the point of the stem turned upwards, so in 

 the leaf is the horizontal, in which its npper, darker-coloured sur- 

 face is turned towards the sky ; into this position it is brought 

 back by the influence of gravity, when it is disarranged, and it is 



