Most remarkable instances of propagation by means of portions of stems are on record. Chips from a tree trunk have been known to produce plants, and the olive is readily increased by knots or excrescences formed upon the trunks of old trees. These excrescences occur in many plants and are known as _knaurs_. They are often abundant about the base of large plane-trees. But they are not often used for purposes of propagation. Whole trunks will sometimes grow after having been cut for many months, especially of such plants as cactuses, many euphorbias and yuccas. And sections of these spongy trunks will grow, also. Even saw-logs of our common trees, as elm and ash, will sprout while in the "boom," or water. _Green-wood cuttings_ are more commonly employed than those from the mature wood, as they "strike" more quickly, they can be handled under glass in the winter, and more species can be propagated by them than by hard-wood cuttings. "Slips" are green-wooded cuttings, but the term is often restricted to designate those which are made by pulling or "slipping" off a small side shoot. All soft-wooded plants and many ornamental shrubs are increased by green cuttings. There are two general classes of green-wood cuttings: those made from the soft and still growing wood, and those made from the nearly ripened green-wood, as in _Azalea Indica_, oleander, ficus, etc. House plants, as geraniums, coleuses, carnations, fuchsias, and the like, are grown from the soft young wood, and many harder-wooded plants are grown in the same way. Sometimes truly hard wood is used, as in camellia. [Illustration: Fig. 51. Tough and brittle wood.]