CHAP. VI. 
OF LEAVES. 
327 
no colouring matter enters them. Considering, however, 
the thinness of the epidermis 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 
which Bonnet contends for. This seems to be proved by the 
effects obviously produced by a, shower of rain in the summer, 
or by syringing the fading plants in a hothouse. 
In their growth, leaves usually increase in length by addition 
to the base, the apex altering but little after it is originally 
formed. Upon this subject there is a paper by Steinheil 
which deserves to be consulted (Ann. Sc., n. s., viii. 257.). 
This is only what theory would necessarily lead to, when it 
is considered that a leaf is an expansion of the epiphloeum 
(see p. 89.) and mesophloeum, its apex representing the ex- 
ternal or ungrowing part of those cortical layers, and its 
base their interior or growing part. 
Leaves usually are so placed upon the stem that their 
upper surface is turned towards the heavens, their lower to- 
wards 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 sur- 
face becomes almost parallel with the stem, and the upper 
surface is far removed from opposition to the heavens. A 
few plants, moreover, 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 sur- 
face becomes uppermost: the former is especially the case 
with plants bearing phyllodia, or spurious leaves. 
At night a phenomenon -occurs in plants which is called 
i 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 
I a single season ; in others, as the Beech and Hornbeam, they 
