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The third section relates to the direction of the leaves of plants. 

 After having described the appearance and structure of the two 

 surfaces of leaves, and mentioned the known fact of the direction of 

 the superior or varnished surface towards the light, the author re- 

 views the labours of Bonnet and Dutrochet on this subject. As no 

 direct experiments had proved that the direction of leaves is due to 

 the influence of light itself, he begins by showing, first, that the 

 turning over of inverted leaves does not take place regularly in com- 

 plete obscurity ; secondly, that it is possible to induce a leaf to turn 

 itself over by screening its superior surface, and by lighting its in- 

 ferior one with an inclined mirror; and thirdly, that when both the 

 surfaces of the leaf are illuminated, the leaf takes a globular form 

 so as to protect the inferior surface from the light. Pie afterwards 

 shows that although in most plants the turning over of inverted 

 leaves takes place through a movement of the footstalk, yet in some 

 of them it is the flat part of the leaf that curls itself over, and that 

 in all the same thing happens when it is the best way of bringing 

 back the superior surface of the leaf towards the light. This is the 

 case, for instance, when the footstalk has been removed, and when 

 the inferior surface is lighted by a mirror and the superior one is 

 screened by a piece of black paper fixed to it. The flat part of the 

 leaf bends its edges and takes a globular form. 



The two surfaces of the leaf do not seem to be under the influ- 

 ence of any real attraction towards the light, for v/hen placed in an 

 inverted position on moveable floats, the leaf turns itself over by 

 means of the footstalk, or the curling of the flat part, without cre- 

 ating any motion in the float. The removal of one or many leaflets 

 in a compound leaf, or of part of a single leaf, does not prevent the 

 turning over of the remainder when placed towards the light in an 

 inverted position. By means of coloured glasses, the purity of the 

 light of which had been ascertained by the prism, it has been shown 

 that the leaves turn over best in the blue rays; next in the violet, 

 but not at all in the red. 



The author next examines the diff'erences produced in the power 

 of leaves to exhale and decompose carbonic acid, according as the 

 light shines on one or other of their surfaces. In order to measure 

 the exhalation, he placed in a bottle full of water of known weight, 

 a leaf with one of its surfaces darkened and the other exposed to 

 light, changing the surfaces alternately while the experiment lasted. 

 The result of a great many series of experiments has been to show 

 that the loss of water by exhalation in all temperatures and by all 

 weathers is much more considerable during the same time when the 

 inferior surface of the leaf is exposed to light, than when the supe- 

 rior surface is so exposed. This increase explains the rapid wither- 

 ing and subsequent death of inverted leaves which cannot turn them- 

 selves over. In coloured glasses the blue rays created the greatest 

 exhalation ; next the diffused white light ; next the green ; the red 

 rays coming last. 



On examining, under the same circumstances, the decomposition 

 of carbonic acid, the author first sought in what part of the leaf this 



