[Illustration: Flax grown for fiber.] Vegetable Oils.--Some of the same plants which give fiber also produce oil. Cotton seed oil pressed from the seeds, linseed oil from the seeds of the flax plant, and coconut oil (the covering of the nut here producing the fiber) are examples. [Illustration: Poison ivy, a climbing plant which is poisonous to touch. Notice the leaves in threes.] Some Harmful Green Plants.--We have seen that on the whole green plants are useful to man. There are, however, some that are harmful. For example, the poison ivy is extremely poisonous to touch. The poison ivy is a climbing plant which attaches itself to the trees or walls by means of tiny air roots which grow out from the stem. It is distinguished from its harmless climbing neighbor, the Virginia Creeper, by the fact that its leaves are notched in _threes_ instead of _fives_. Every boy and girl should know poison ivy. Numerous other poisonous common plants are found, but one other deserves special notice because of its presence in vacant city lots. The Jimson Weed (_Datura_) is a bushy plant, from two to five feet high, bearing large leaves. It has white or purplish flowers, and later bears a four-valved seed pod containing several hundred seeds. These plants contain a powerful poison, and people are often made seriously ill by eating the roots or other parts by mistake. Weeds.--From the economic standpoint the green plants which do the greatest damage are weeds. Those plants which provide best for their young are usually the most successful in life's race. Plants which combine with the ability to scatter many seeds over a wide territory the additional characteristics of rapid growth, resistance to dangers of extreme cold or heat, attacks of enemies, inedibility, and peculiar adaptations to cross-pollination or self-pollination, are usually spoken of as weeds. They flourish in the sterile soil of the roadside and in the fertile soil of the garden. By means of rapid growth they kill other plants of slower growth by usurping their territory. Slow-growing plants are thus actually exterminated. Many of our common weeds have been introduced from other countries and have, through their numerous adaptations, driven out other plants which stood in their way. Such is the Russian Thistle. A single plant of this kind will give rise to over 20,000 seeds. First introduced from Russia in 1873, it spread so rapidly that in twenty years it had appeared as a common weed over an area of some twenty-five thousand square miles. It is now one of the greatest pests in our Northwest. REFERENCE BOOKS ELEMENTARY