The thistle of the meadow is as safe from hungry cattle as though fenced in by barbed wire. A cow must be starving that would care to flavor her luncheon with the needles that the thistle bears. The common skunk cabbage would make a tempting meal for her after a winter of dry feeding, had not Nature given it an odor that disgusts even a spring-time appetite. The milkweed welcomes the bees and flies that help to distribute her pollen where she wants it spread, but she has her own way of punishing the useless thieves that trespass up her stalk. Wherever the hooks of an insect's feet pierce her tender skin, she pours out a milky juice to entangle its feet and body, and it is a lucky bug that succeeds in escaping before this juice hardens, and holds him a prisoner condemned to die. All over the world there are plants with the same ability that the milkweed has, but it is especially true of certain trees and vines of the tropics. As soon as the little beetle begins to bore into the bark of one of these trees, there pours out a sticky, milky fluid that kills the insect at once. If this were all, the wound would remain open, ready for the next robber who came along. In order that the break may be healed, a cement is necessary, but not a hard, unyielding one, for that would crumble away with the motion of the tree in the wind. So with Mother Nature's perfection in doing things, the very plant juice that has done duty as a poison is hardened into an elastic stopper, with the result that, no matter how far the tree may sway and tug at the wound, the filling gives and stretches, true to the task it has to perform. This was the juice the crafty savage induced the tree to give up. Wherever the bark was cut, the fluid poured forth to heal the break and hardened like blood on a cut finger. The native caught it while it was still soft and applied it to his simple needs. This juice is not the sap of the rubber tree. Sap, which is the life-blood of the tree, flows through the wood, but the juice we are describing is contained in the inner bark, a thin layer directly below the outer bark. Scientific men call this juice latex. It is like milk in three ways: it is white, it contains tiny particles that rise to the top like cream, and it spoils quickly. The particles in cow's milk are full of fats which make it good for us to drink. But a rubber tree's milk has tiny atoms of rubber and resin and other things, and it took time to discover which of the vines and trees was the prize milker of the tropics and gave the largest amount of pure rubber. Finally, the Hevea, the very tree the Frenchman wrote about, proved to be the best, and, although by no means the only rubber tree of commercial value, it is acknowledged the greatest of rubber trees.