Besides the spiral vessels that are attached to the interior walls of the vascular ducts, there are groups of independent spiral vessels of great beauty and elasticity, of which the seeds of the wild clary afford a remarkable instance. They consist of cylindrical tubes with conical extremities twisted into a right or left-handed screw, which can be unrolled without breaking. They are found in the leaves of almost all plants, in the petals and stamens of flowers, in the stalks of all fruits, even in the minutest seeds; large parallel bundles of them imbedded in hexagonal cellular tissue may be seen in the veins of the kernel of the hazel nut, and they constitute the medullary sheath which surrounds the pith in trees. They are all hollow, and capable of conducting liquids. [Illustration: Fig. 5. Longitudinal section of stem of Italian reed:—_a_, pith; _b_, fibro-vascular bundles; _c_, cuticle.] The laticiferous vessels or vasa propria, those which contain the proper juices of plants whether milky or coloured, are exceedingly varied in their forms and arrangement in different plants, and in different parts of the same plant. In the leaves they generally form a delicate capillary network, in the bark they constitute a system of long vascular ducts forming an elongated irregular network pervious to the proper juices throughout; sometimes they are formed of cells joined end to end, and frequently they are thin branching flexuous tubes, meandering through the passages or interstices of the cellular tissue, and occasionally filling the lacunæ.[31] Every one of the preceding tissues may be found in many of the highest class of vegetables—those which are distinguished by having seeds with two lobes or seed leaves, such as our common trees, shrubs, and most of the herbaceous plants. Palms, the cereals, grasses, canes, and all plants having seeds with but one lobe, which form the second class, consist of cellular tissue mixed with fibro-vascular bundles; whilst in the third or flowerless spore-bearing class, there is a general tendency to a more and more simple structure, from the tree fern to the lichens and algæ, which last consist of cellular tissue alone, and contain the lowest germs of vegetable life.