All the plants which count caoutchouc among their products belong to the torrid zone. A. von Humboldt, in his _Ideas of a Geography of Plants_, remarked that the plants yielding _milky_ juices multiply as we approach the tropics. This _milky juice_ of plants it is which contains the peculiar elastic substance. The tropical heat seems to exert a distinct influence in its perfect formation, for it has been remarked that the same plants which under the equator yield abundance of caoutchouc contain instead, with us, even in hothouses, a substance which resembles the bird-lime obtained from our native mistletoe. Who among my readers has not seen our indigenous wolf’s-milk or spurge, the white milky juice of which popular superstition recommends as a remedy against warts? Who has not in youth at least become acquainted with the celandine, from the broken stalk and leaf of which a bright orange-colored juice runs out? Who has not observed that the lettuce, when it has run up to flower, ejects a milk-white fluid at the slightest touch? But the occurrence of milky juices in plants is not limited to these few. The vegetable world presents to us most useful as well as poisonous matters in this milky sap, and I will content myself at present with recalling to recollection opium, the dried milky juice of our large garden poppy. A great number of plants, which principally belong to three great families, namely, the Spurges, the Apocynoceæ, and the Nettle plants, are distinguished by a peculiar anatomical structure. In their bark, and also partly in their pith, we find a quantity of long, variously curved and branched tubes, which are not unlike the veins of animals. In these tubes we find a thick juice of the consistence of very rich milk, whence it is called milk-sap. Its color is usually milk-white, but yellow, red, and, very rarely, blue milk-saps are met with, but more frequently still they are wholly colorless. Like animal milk, this juice consists of a colorless fluid and small globules. The composition displays the most varied constituents, and upon the variation of quantity and modes of mixture of these matters depend the abundant varieties of this juice. All contain more or less caoutchouc, which occurs in the form of little globules. These are prevented from coalescing by an albuminous substance, in the same way as are the butter globules in milk. Exactly like the cream (the butter) in milk, the caoutchouc globules rise to the surface of the milk-sap of plants when left to stand, here form a cream, and can not, any more than butter, be separated again into their distinct globules. All those three great families which are distinguished by their abundance of milk-sap, although differing very widely botanically, exhibit some most remarkable agreements through the nature of their milk-sap.