VEGETABLE PHYSIOLOGY.-THE LEAVES OF PLANTS. 27 
what is called the sleep of plants; the folding of the leaflets in 
many plants, and the opening and shutting of the blossoms in 
others, are all to be referred to the action of light and of the at¬ 
mosphere. This phenomenon is moreover attested by the action 
of poisons on vegetables, which is similar to that produced on 
the animal structure ; this has been frequently proved by expe¬ 
riment. 
Spontaneous movement is another kind of irritability pos¬ 
sessed by some plants : thus, in Megaclinium falcatum the 
labellum is almost continually in motion; and a kind of convul¬ 
sive action of the same part may be observed in some species 
of Pterostylis; the filaments of Oscillatoria are constantly 
writhing like worms in pain ; and in Hedysarum gyrans the la¬ 
teral leaflets are in motion both day and night, even when the 
terminal one is asleep : the leaves of the aspen also furnish a 
familiar illustration of spontaneous movement, for, by a curious 
formation of the petiole, they can never rest, but are in con¬ 
stant motion. 
The fall of the leaf is a phenomenon too important to be 
here passed over : it is explained by De Candolle, thus : -— the 
increase of the leaves both in width and length, that is, their 
full expansion, is generally a rapid process ; for a time the 
leaves exercise their varied functions, but all this while the pro¬ 
cess of denudation is silently going on. They exhale perfectly 
pure water, and retain in their tissue the earthy matter carried 
up by the sap; and in consequence of this the vessels harden 
and the pores become obstructed : this goes on during the sea¬ 
son of their growth ; and according as evaporation is more or 
less active, so are the leaves approaching the limit of their 
existence : they gradually dry up and die. But this death of 
the leaves must not be confounded with their falling, for the 
two processes are distinct : the death ot the leaves results fiom 
the choking up of their vessels; they then change colour, cease 
to decompose carbonic acid and absorb oxygen, become un¬ 
healthy, and die; but the living tissue at their base still con¬ 
tinues to increase in size, and thus the dead or dying leaves 
are, as it were, forced outwards and thrown off. Du Petit 
Thouars gives another explanation 44 If,” he says, 4 ‘we watch 
the progress of a tree, we shall perceive that the lowest leaves 
fall first;” and this he explains thus: — 44 The base of every leaf 
reposes on the pith of the branch, to the sheath of which it is 
e 2 
