290 ANNUAL OF SCIENTIFIC DISCOVERY. 



conclusion. He found that, however long it might be necessary for a 

 stem to grow in order to reach the light, its base attached to the float 

 always remained on the same spot. In one instance, a germinate seed 

 of mustard having been placed on a float in a tumbler surrounded by 

 dark paper, but near an aperture admitting luminous rays, the plant 

 put forth a stem, which passed all round the tumbler to spread its 

 leaves in the part of the vessel in which was the luminous aperture ; 

 once there it did not extend itself beyond it, but grew erect, although 

 the light was not strong enough to render it entirely green. Thus, 

 although a slight motion of the float would have brought the entire 

 plant within range of the light, its position remained wholly un- 

 changed. The observations of Prof. Macaire are opposed to the 

 hypothesis of De Candolle in this case, as in the preceding, since he 

 found that the stems grew straight towards the light, without the in- 

 curvation or bending which that hypothesis assumes. Where young 

 plants already vigorous were placed on the floats in the dark portion of 

 the vessel, their green stems took little or no ulterior development, 

 but from the neck of the root there grew out another stem, white and 

 etiolated, which spread itself along the water to reach the light por- 

 tion of the vessel, where it grew erect, and put forth its leaves. 



The next subject examined was the direction of the leaves, that is, 

 the tendency of those which have two surfaces of different hues to 

 expose the deeper-colored to the sky, and the paler to the earth. Prof. 

 Macaire's experiments lead him to the same conclusion that other 

 physiologists have come to, that light is the only agent in turn- 

 ing over the leaves, and that it does not act by a physical attraction, 

 properly so called, but by its influence upon the individual parts of the 

 tissues on which it falls. Tin's influence is the more rapid and ener- 

 getic, all other circumstances being alike, the greater the difference 

 between the two surfaces of the leaves experimented on. It was 

 maintained by Bonnet and Dutrochet, that the turning over of the 

 leaves always takes place by a flexion or tension of the footstalk ; but 

 Prof. Macaire has demonstrated, that the flat portion of the leaf, or 

 even a separate portion of it, can turn itself over. Thus, when an en- 

 tire branch of geranium was immersed in water in such a way as to 

 expose the under surface only of its leaves to the light, all the young 

 leaves turned themselves over in three days, by moving on the point 

 of insertion of the flat part of the leaf into the footstalk ; and in other 

 experiments, in which, by means of a screen, the light was prevented 

 from falling upon the upper surface of the leaves, and by a mirror was 

 directed to the lower, the margins of the leaves bent clown in such a 

 manner as to bring their upper surfaces within the influence of the 

 mirror. Upon repeating these experiments, with glasses of different 

 colors, it was found that the leaves turned over most readily in blue 

 rays, and next in violet, but that they remained motionless in red. 



Prof. Macaire next inquires experimentally, how far these results 

 are attributable to the influence of light on the nutritive functions, 

 in which the leaves are concerned, and comes to the conclusion 

 that their explanation is to be sought here. He found that the exha- 

 lation of fluid from the leaves is always greatly augmented by the ex- 



