There are two other rubber-producing plants, neither of which are as important as _Hevea_ and _Castilla_. One is the common rubber plant so much grown as a house plant and said to be much cherished for that purpose in Brooklyn. It is a kind of fig tree, known as _Ficus elastica_, a native of India and the source of India rubber. It was used for years, before the days of vulcanization, mostly for lead-pencil erasers. Thousands of acres of it in India were recently destroyed to make room for _Hevea_, although it still produces a respectable amount of rubber, inferior, however, to _Hevea_ and _Castilla_. In Mexico a new source of rubber is the guayule rubber plant, a small shrub native of the drier parts of the Mexican uplands. It does not produce latex, as practically all other rubber plants do, but particles of rubber are found directly in its tissues, mostly in the bark. While of some importance, this shrub, known as _Parthenium argentatum_, a member of the _Compositæ_ or daisy family, is not likely to become a dangerous rival of either Para or _Castilla_ rubber. Wild sources of guayule rubber are already reported as diminishing rapidly, so that its permanent success will depend upon cultivation. At least a score of other plants are known to produce rubber of a kind, but none of them have yet been much developed. From most plants with a milky juice, such as dogbane or spurge, rubber of some sort can usually be recovered. The production of this substance is directly due to a response of the plant to wounds, and there are still great fields of research necessary on this phase of plant activity, upon which a great industry has already been built. 5. DRUGS Nearly all the drugs and medicines of importance are of vegetable origin, and from the days of Theophrastus the study of plants as possible medicines has been one of the chief phases of botanical research. In the early days all that was known about plants was learned by men interested in medicine, and some of their quaint old books are interesting relics of a bygone day. At present, pharmacognosy, or the science of medicinal plant products, is a highly developed specialty taught in medical and pharmacy schools. And yet the greatest medical college in this country has recently issued instructions to its staff of doctors and nurses to pay particular attention to “old wives’ remedies,” most of which consist of decoctions of leaves and other parts of plants. They have done this because all the knowledge of the scientists regarding medicinal plants has its origin in the habit of simple people turning to their local plants for a cure. The accumulation of the ages, aided and guided by the scientist, has resulted in the wonderful things that can now be done to the human body through the different drugs, nine-tenths of which are of plant origin.