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POPULAR SCIENCE REVIEW. 
that its lower surface faces outwards. The upper surfaces of 
the opposite leaflets are thus brought into contact with one 
another beneath the petiole, and are well protected, as shown 
in the woodcut.* The rotation and other movements are 
effected by means of a well- developed pulvinus at the base of 
each leaflet, as could be plainly seen when a straight, narrow 
black line had been painted along it during the day. The two 
terminal leaflets in the daytime include rather less than a right 
angle, but their divergence increases greatly whilst they sink 
downwards and rotate, so that they stand laterally at night, as 
may be seen in the figure ; moreover, they move somewhat 
backwards, so as to point towards the base of the petiole. In 
one instance, Mr. Darwin found that the mid-rib of a terminal 
leaflet formed at night an angle of 36°, with a line dropped 
perpendicularly from the end of the petiole. The second pair 
of leaflets likewise moves a little backwards, but less than the 
terminal pair ; and the third pair moves vertically downwards, 
or even a little forwards. Thus all the leaflets in those species 
which bear only three or four pairs, tend to form a single 
packet, with their upper surfaces in contact and their lower 
surfaces turned outwards. Lastly, the main petiole rises at 
night, but with leaves of different ages to very different 
degrees ; thus some rose through an angle of only 12°, and 
others as much as 41°. 
The influence of light upon the movements of plants is 
various. Thus the so-called heliotropic movements are deter- 
mined by the direction of the light, whilst periodic movements 
are affected by changes in its intensity. On the other hand, 
apheliotropism implies that a plant bends from the light, a 
rare phenomenon, at least in a well-marked degree. Parts of 
plants under the influence of diaheliotropism place themselves 
more or less transversely to the direction whence the light 
proceeds, and are thus fully illuminated. Lastly, some leaves 
rise or sink or twist so as to avoid great intensity of light. 
Such a phenomenon, Mr. Darwin suggests, should be called 
paraheliotropic. All these movements consist of modified 
circumnutation. Space forbids a detailed description of these 
effects ; but one curious result of Mr. Darwin’s investigation 
is worth recording, and that is, the transmitted effects of light. 
When, for example, the cotyledon of Phalaris canariensis is 
exposed to light, the upper part bends first, and afterwards 
the bending gradually extends to the base, and even a little 
below the ground. By protecting the whole upper half of the 
cotyledon from light, the lower part, though, fully exposed to 
light, was prevented from becoming curved. Hence it is to be 
* I am indebted to the courtesy of Mr. Darwin for the loan of the blocks 
for these figures. 
