The most familiar examples occurring among our native plants are furnished by the trifoliate leaves of many of the clover family. As evening approaches, the clovers and their allies fold their three leaflets together by means of an upward movement; the juxtaposition of the leaflets retards loss of heat, and the vertical position which they thus assume has the same effect, tending to check the radiation of heat to the cold sky overhead. The wood sorrel, which, though of a quite different order, has leaves which resemble those of the clovers, effects the same object by folding its leaflets _downward_. Wet, which by lying on the leaves might hinder transpiration, must also be guarded against; a danger which in many species is obviated by means of a waxy excretion, especially on those parts of the leaves where the stomata are situated; on which, as on an oily surface, water will not lie. Another danger to which plants are exposed, and one which we might think they would be powerless to meet, is the attacks of browsing animals--animals of all sizes, from minute insects up to great munching cattle. But to note how perfectly such defence may be provided for we need only look at our common gorse, which boldly invades the pasture, protected by its impenetrable chevaux-de-frise. This plant, indeed, seems to have put so much of its vital energy into the production of spines that it has none left with which to produce leaves, and the making of plant-food has to be carried on by the green and much-branched stems. The beautiful tribe of the thistles naturally comes to our minds in this connection. Armed with innumerable spines of the most exquisite structure, sharper and more delicate far than needles, the spear thistle and marsh thistle raise their tall and graceful forms untouched amid the close-browsed herbage, and without fear of molestation--save from man, with his implements of iron--open their flower-heads to the sun and the insects, and scatter their numberless winged fruits to the wind. In the thistle the spines are borne alike on the stems, leaves, and involucres or outer whorls of the heads of flowers. The holly is an interesting case. In low bushes the edges of the leaves are provided with strong spines; but when the bush grows into a tree, and bears leaves far above the reach of browsing animals, the unnecessary spines disappear, and the edges of the leaves are entire. In the blackthorn and hawthorn, the strong spines are modified branches; and we may observe that they are much more numerous in young plants than in old bushes. A more complicated mode of protection is found in the nettles. They are furnished with hollow hairs, filled with a virulent fluid, and bent at the tip. A slight pressure causes the curved extremity to break across, leaving a slender tube, tapering to an extremely fine point, which easily enters the flesh and discharges a portion of its venomous contents.