CHAP. IV. OF LEAVES. 271 



fusions, no colouring matter enters them. Considei'ing, how- 

 ever, the thinness of the cuticle of many plants, and the gi-eat 

 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 

 oflP: this happens at extremely unequal periods in different 

 species. In some they all wither and fall oflF 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 : — 



