FUNCTION.] POSITION OF LEAVES. 205 



in fact, no power of attracting fluid. In proof of this it is 

 urged, that, if leaves are made to float on coloured infusions, 

 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 must be 

 admitted that they do possess the power of absorption which 

 Bonnet contends for. This is sufficiently proved by the 

 effect obviously produced 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 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 

 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. 



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 last are our evergreens. Mirbel distinguishes 



