Highly specialized air-sacs are characteristic of all birds.
To this category belong certain sacs and pouches.
Air-sacs, by which the breathing capacity is much increased.
In some cases special secreting tissues, resin ducts, oil glands, laticiferous tissue, crystal sacs, &c., may be developed among the ordinary secondary vascular elements.
The formation of the conducting tubes or secretory sacs which occur in all parts of the higher plants is due either to the elongation of single cells or to the fusion of cells together in rows by the absorption of the cell-walls separating them.