PHYSIOLOGY. — 808 
tiie plant. When they fall off, they have come to 
their greatest age, and the plant can exist for some 
time without them. ‘The dead leaves separate from 
the living part, like dead Sa in the animal eco- 
nomy fie sound ones. 
Were the opinions of Du Hamel and Mustel 
founded in truth, the leaves would never fall off in 
warm climates. But there are in the East Indies 
some trees which, at the rainy season, drop all their 
leaves, and like our’ trees, are pertectly leafless. 
Mr Thunberg likewise saw at Java an oak tree 
which lost its leaves at the Same time as in Europe. 
Theré must therefore exist another cause of this 
phenomenon. Vrolick’s opinion is just, and Ber 
fectly corresponds with all observations. 
The true cause of the falling off of the leaves is 
this : During the summer, the vessels of the petiole 
become gradually ligneous, as the sap is conveyed 
to them in greater quantity, and the whole frame of 
the leaves gets a more ligneous consistence. The 
sap must In consequence stagnate, and at last the 
commiunicating substances between the stem and the 
petiole are completely dried up andcrack. ‘Thewound 
which the stem thus reveives cicatrizes before the 
petiole separates. The connexion now interrupted 
between the leaf and the stem, and their: vessels, 
causes the petiole, by which they are connected, to 
separate entirely, and thus, in calm serene weather 
especially, the leaves unavoidably fall off. Be- 
sides, the rays of the sun still favour the last de- 
composition of the water, but the reducent vessels 
xannot convey the small quantity of moisture to the 
i) knot 
