Air-sacs, by which the breathing capacity is much increased.
The spores, as in the heterosporous Pteridophyta, are of two kindsmicrospores (pollen grains) borne in microsporangia (pollen sacs) on special leaves (sporophylls) known as stamens, and macrospores (embryo-sac) borne in macrosporangia (ovules) on sporophylls known as carpels.
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.
The ventral portion of the neck is formed by the strong crista inferior, on the median side of which is the deep fosses subtrochanterica by which air sacs enter the humerus.
Exchange of gas through the walls of the air-sacs, almost devoid of blood-vessels, can at best be much restricted.