SPIRAL VESSELS are those very delicate air-tubes in the cellular tissue of plants which run unbranched through the different parts of the plant, and whose walls are composed of fibers spirally or circularly twined. Spiral vessels are eitherfree,when their windings are unconnected with each other, or when the windings are involved with each other in a net-like manner. If the free spaces between the convolutions in the latter are linear, they form lined vessels; but if they are point-like, they form punctate or porose vessels. Spiral vessels, whose walls are formed of distinct horizontal rings, placed simply one above another, are called annular vessels. Spiral vessels seldom occur singly, but are generally united by cells into bundles called vascular bundles. These vascular bun dles are scattered in the stems of endogenous plants; but in the stems of oxogenous plants they are arranged in one or more concentric circles. Among cryptogamous plants, the ferns alone (in the most extensive signification of the term) are provided with spiral vessels. All plants which have spiral vessels are called vascular plants, in contra
distinction to cellular plants, whose substance consists of cells only.
Through the operation of what laws thespiral form is assumed by spiral vessels: is still unknown, although the question has naturally been regarded as having an intimate con nection with the tendency to spiral structure manifested in plants, and even in some of those cryptogamous plants in which no true spiral vessels are found; a tendency which is observed not only in spiral stems, spiral tendrils, the spiral fibers of the elaters of jungermannite, and the like, but throughout the vegetable kingdom generally in the spiral arrangement of leaves and of the organs which are formed by the metamorphosis of leaves. The whole subject is an extremely difficult one; there has been much specu lation about it, but as yet with no satisfactory results.