198 
POPULAR SCIENCE REVIEW. 
tion. As Mr. Darwin observes that only a few of the flower- 
heads, which from their position are not able to reach the 
ground and bury themselves, yield seeds, whereas the buried 
ones never failed to produce as many seeds as there had 
been perfect flowers, it may be reasonably conjectured that 
the object gained is to nourish the ripening seeds directly 
through their surfaces, and so to supplement root action. The 
capsules of Cyclamen and of the Wood Sorrel, Oxalis acetosella, 
are occasionally buried, but then only beneath dead leaves or 
moss. 
Leaves. — The movements of certain leaves which are said 
to sleep, have long been observed, but it would seem that 
probably all leaves and cotyledons circumnutate, and that the 
so-called sleep is only a remarkable modification or development 
of this general kind of movement, accompanied, however, by 
other and often complicated motions. The seat of the move- 
ment generally lies in the petiole, but sometimes both in the 
petiole and blade, or in the blade alone. The movement is 
chiefly in a vertical plane ; but as the ascending and descending 
lines never coincide, there is always some lateral movement, 
and thus irregular ellipses are formed. There is a periodicity 
in the movements of leaves, for they often or generally rise a 
little in the evening and early part of the night, and sink 
again on the following morning, this periodicity being 
determined by daily alternations of light and darkness, as 
already mentioned in the case of cotyledons. These periodic 
movements occur where there is no pulvinus, for where this is 
found, the movement is amplified into nyctitropic , or sleep- 
movements. 
Leaves, Mr. Darwin says, when they go to sleep, move 
either upwards or downwards ; or in the case of the leaflets of 
compound leaves, forwards, that is, towards the apex of the 
leaf, or backwards, that is, towards its base ; or again, they 
may rotate on their own axis without moving either upwards 
or downwards ; but in almost every case the plane of the blade 
is so placed as to stand nearly or quite vertically at night. 
Moreover, the upper surface of each leaf, and more especially 
of each leaflet, is often brought into close contact with that of 
the opposite one, as the upper surfaces appear to require more 
protection than the lower. 
The nyctitropic movements of leaves and cotyledons are 
effected in two ways, firstly, by means of pulvini, which become 
ultimately more turgescent on opposite sides ; and secondly, by 
increased growth along one side of the petiole or mid- rib, and 
then on the opposite side. This difference between the two 
means of movement consists chiefly in the turgescence of the 
cells of a fully-developed pulvinus not being followed by 
