FUNCTIONS OF LEAVES. 75 



Where the foot-stalk of the leaf has to bear a considerable 

 weight, and is moreover very much exposed to the wind, addi- 

 tional precautions have been taken for increasing its strength. 

 Thus, the foot-stalks of the huge fronds of the cocoa-palm are 

 inclosed in a tough web or network, which preserves them so well 

 from breaking, that even after death they remain attached to the 

 tree. In the flexible grasses we find the leaves embracing the stem 

 with a sheath, which gives to both a much greater power of re- 

 sistance, while in many herbs the sessile leaves are placed in such 

 a manner that the rain or dew collecting on their surface flows 

 down the stalk to the roots, where it is most needed. In several 

 aquatic plants, the stalks of the leaves are ventricosely distended, 

 so as to render them buoyant, and in many of the fuci, the large 

 air-vessels with which the stem or the fronds are furnished, 

 answer a similar purpose. These few examples sufficiently prove 

 that it is not by a mere caprice of growth that some leaves are 

 barely suspended from stalks, while others embrace the stem of 

 the plant, but that every variety of form is made to answer 

 an especial end. No plant has been neglected, none has been 

 encumbered with useless or unappropriate organs, but each has 

 received all that it required. 



Some leaves have been gifted with a wonderful sensibility which 

 seems almost to raise them to the level of animal life. Thus the 

 Porliera hygrometrica foretels serene or rainy weather by the 

 opening or closing of its leaves. Large tracts of country in Brazil 

 are almost entirely covered with sensitive plants. The tramp 

 of a horse sets the nearest ones in motion, and, as if by magic, 

 the contraction of the small grey-green leaflets spreads in quiver- 

 ing circles over the field, making one almost believe with Darwin 

 and Dutrochet that plants have feeling, or tempting one to 

 exclaim with Wordsworth 



It is my faith that every flower 

 Enjoys the air it breathes. 



The leaves of the Venus's Fly-trap (Dioncea muscipula), a 

 marsh-plant of North America, are still more curious, as their 

 wonderful contractility gives them an offensive power quite 

 unique in the vegetable world. They are oblong, and divided 

 by the mid-rib into two halves inclining" towards each other, 

 and beset on the upper surface and along the edges with 

 long and stiff bristles. At the slightest touch the two halves 



