CHAP. IV. 
OF LEAVES. 
271 
fusions, no colouring matter enters them. Considering, how- 
ever, the thinness of the cuticle of many plants, and the great 
permeability of vegetable membrane in general, it can hardly 
be doubted that they do possess the power of absorption that 
Bonnet contends for. This seems to be further proved by 
the respective effects obviously produced upon plants by a 
shower of rain in the summer, or by syringing the fading 
plants in a hothouse. 
Leaves usually are so placed upon the stem that their upper 
surface is turned towards the heavens, their lower towards the 
earth ; but this position varies occasionally. In some plants 
they are imbricated, so as to be almost parallel with the stem ; 
in others they are deflexed till the lower surface becomes 
almost parallel with the stem, and the upper surface is far re- 
moved from opposition to the heavens. A few plants, more- 
over, invert the usual position of the leaves by twisting the 
petiole half round, so that either the two margins become 
opposed to earth and sky, or the lower surface becomes up- 
permost : this is especially the case with plants bearing phyl- 
lodia, or spurious leaves. 
At night a phenomenon occurs in plants which is called 
their sleep : it consists in the leaves folding up and drooping, 
as those of the sensitive plant when touched. This scarcely 
happens perceptibly except in compound leaves, in which the 
leaflets are articulated with the petiole, and the petiole with 
the stem : it is supposed to be caused by the absence of light, 
and will be farther spoken of under the head of irritability. 
After the leaves have performed their functions, they fall 
off : this happens at extremely unequal periods in different 
species. In some they all wither and fall off by the end of a 
single season ; in others, as the beech and hornbeam, they 
wither in the autumn, but do not fall off till the succeeding 
spring; and, in a third class, they neither wither nor fall off 
the first season, but retain their verdure during the winter, 
and till long after the commencement of another year's 
growth : these are our evergreens. Mirbel distinguishes 
leaves into three kinds, as characterised by their periods of 
falling : — 
