"And the thorns which make us think Of the thornless river brink Where the ransomed tread".
- Mrs. Browning.
The Book of Genesis teaches that thorns and thistles grew out of the cursed ground, in punishment for our first parents' sin. Modern science, harmonizing with ancient theology, holds that thistles and nettles, as we know them to-day, are younger children in the great family of plants. They are "highly specialized".
The larger thistles are suited in color and structure to the tastes and needs of the bumble-bees, which are among the latest born of insects. The nettles employ the wind as their pollen-carrier, and are wondrously adapted to make the best use of this capricious servant, which outdoes the most exacting of trades unions in its determination to "lay off' when it pleases and to regulate its own holidays and the length of its working-day. And both thistles and nettles are guarded, with Nature's utmost care, against pollen-thieves and grazing foes.
Fig. 89. - Nettle and Canada thistle (Urtica dioica and Cnicus arvensis).
Both have juicy stalks, and leaves toothsome to vegetarian rovers. Both grow in uncultivated fields, along roadsides, and in waste open spots where grass is scarce, and where hungry cows and holiday-keeping horses are wont to wander, seeking what they may devour.
The thistle is saved from those who would eat it up by a bristling armor of prickles, dismaying to all animals except the donkey. That proverbially determined quadruped will not be turned from his gastronomic purpose by little things like these. Indeed, he seems to relish them as a pungent addition, giving zest to his repast, much as cayenne pepper and Chili sauce improve the dinner of the human gourmet. But to most animals the prickles of the thistle and the stings of the nettle are hurtful and repellent. "Weeds or shrubs with juicy, tender leaves," says Grant Allen, "are very apt to be eaten down by rabbits, cows, and other wandering herbivores. But if any individuals among such plants develop any peculiarities which prevent animals from browsing upon them, then those particular plants will be spared, while their neighbors are eaten. They will live to produce offspring inheriting the habits of their parents, and of these offspring the more tender and defenceless will be eaten, while the thorniest, stringiest, or bitterest individuals will be spared, to produce offspring thorny, stringy, and bitter, like themselves. So, in the course of generations, Nature brings into being a number of plant-families, each protected from browsing animals by some well-marked peculiarity".
The common mullein, a plant of the roadsides and pastures, is rendered unappetizing by the down which covers its leaves, and which, it seems, is doubly useful. For "hairs," says Vines, "often serve to diminish transpiration and radiation, and to screen chlorophyll from too intense light, and a clothing of hairs is characteristic of plants which habitually grow in dry soils and in sunny situations." But in this case the fuzz which clothes the mullein-leaves makes them as "dry eating" as so much flannel.
The great cool leaves of the burdock are bitter and sour exceedingly (Fig. 90). So efficient are these devices of Nature for the protection of thistles, mullein, and burdock, that they are generally spared, even in close-cropped pastures. The dandelion and wild lettuce-leaves contain a bitter juice.
Fig. 90. - Burdock (Arctium Lappa).
In the hawthorne, the locust, and the wild orange-tree some of the lower branches develop into sharp spines which prick the noses of would-be assailants.
In the bramble those hairs which clothe the stem of most plants have thickened into pointed prickles. In the holly the angles of the leaves have grown into needle-like points, which deter animals from browsing upon them, and it is noticeable that when the holly develops into a tree its foliage, carried up into comparative safety, becomes almost smooth.
On the Irish gorse, a native of commons where cattle wander, and of mountain-slopes where half-starved sheep run wild, all the leaves are thorns. The green color of these thorns shows that they contain chlorophyll, and they fulfill the office of the foliage, which they have entirely supplanted. The whole gorse-bush, from its root to its crown of honey-sweet golden flowers, is one bristling defiance.
The teasel (Fig. 91) is evolving its armor, which is already disconcerting to a browsing vegetarian, and which may become positively deterrent in times to come. Its leaves are supported by strong, midribs, each of which bristles all down its length, with saw-teeth. The side-veins are studded with smaller teeth, and, while the lower sides of the leaves are thus effectually protected, their upper sides are not always left unguarded. About one teasel in three has its upper leaf-surfaces dotted over with prickles, not very sharp to the fingers, but probably well able to hurt the lips and tongue of a browsing animal, and these prickles, by the bye, are interesting to the botanist, because they grow out of the leaf-surface, and not, after the usual habit of prickles, from the veins. A hungry rabbit, feeding among the teasels, would be likely to satisfy his appetite with those leaves having their upper surfaces soft and smooth, and to spare the more bristly individuals. So these well-guarded plants survive to set their seed, and become progenitors of young plants, which will inherit the parental habit of bearing leaves with prickles on both surfaces.