Annular ducts (Fig. [457]) are marked with cross lines or rings, which are thickened portions of the cell-wall.
Fig. 453, 454. Spiral ducts which uncoil into a single thread. 455. Spiral duct which tears up as a band. 456. An annular duct, with variations above. 457. Loose spiral duct passing into annular. 458. Scalariform ducts of a Fern; part of a bundle, prismatic by pressure. 459. One torn into a band.
Spiral ducts or vessels (Fig. [453-455]) have thin walls, strengthened by a spiral fibre adherent within. This is as delicate and as strong as spider-web: when uncoiled by pulling apart, it tears up and annihilates the cell-wall. The uncoiled threads are seen by gently pulling apart many leaves, such as those of Amaryllis, or the stalk of a Strawberry leaflet.
Fig. 460. Milk Vessels of Dandelion, with cells of the common cellular tissue. 461. Others from the same older and gorged with milky juice. All highly magnified.
Laticiferous ducts, Vessels of the Latex, or Milk-vessels are peculiar branching tubes which hold latex or milky juice in certain plants. It is very difficult to see them, and more so to make out their nature. They are peculiar in branching and inosculating, so as to make a net-work of tubes, running in among the cellular tissue; and they are very small, except when gorged and old (Fig. [460, 461]).