AIR-BLADDER. 



ALARTA. 



1878 ; Beauregard & Galippe, Guide fyc. 

 1880, p. 764 ; Robin, Micr. 1877, p. 870. 



AIR-BLADDER of Fishes. See SWIM- 

 MI XG BLADDER. 



AIR-BUBBLES. See Am. 



AIR-CELLS or Sacs of animal?. These 

 are dilatations or expansions of the air- 

 passages. 



In birds, these are membranous cavities 

 communicating with the lungs and dis- 

 tributed through the chest and abdomen. 

 They extend over almost all parts of the 

 body, around the joints of the extremities, 

 into the bones, the quills, and the feathers, 

 and even between the skin and subjacent 

 muscles. During inspiration, the air enters 

 all these cavities. 



In insects, the air-cells or sacs consist of 

 dilatations of the tracheae. See TRACHEJE. 



Their obvious use is either to diminish 

 the specific gravity of the body, or to act as 

 reservoirs of air during the impeded respi- 

 ration connected with flight. 



BIBL. Siebold and Stannius, Lehrb. d. 

 vert/I. Anat. ; Owen, Hunt. Led. ; Carpenter, 

 Compar. Anat. ; Gegenbaur, Vergl. Anat. 

 1878. 



AIR-PASSAGES in plants are large 

 intercellular passages occurring especially 

 in the stems of Monocotyledons and in the 

 leaves and stems of aquatic plants. Their 

 form and arrangement are sometimes very 

 regular and elegant, especially when they 

 depend upon a certain regular peculiarity of 

 shape in the cells which form the walls of the 

 passages. Thus cross sections of the com- 

 mon rush are pleasing microscopic objects, 

 exhibiting regular stellate cells, the rays of 

 which are separated by large air-passages, 

 giving the spongy texture to the structure. 

 Large air- passages, communicating with the 

 stomata, are not unfrequently lined by a 

 cuticular layer similar to that found upon 

 the external surface of epidermal cells. In 

 the Nymphceacece (Water-lily Order) the 

 large air-passages in the floating leaves and 

 the stem have peculiarly developed star-like 

 cells projecting freely into these cavities; 

 these cells are filledVith a granular sub- 

 stance very unlike the contents of the large 

 cells of the general parenchyma of the leaf. 

 Their nature and office are yet unknown. 

 The partitions separating the air-cells hori- 

 zontally in Limnocharis PlumierifmdAlisma 

 plantaoo form beautiful microscopic objects. 

 The stems of the Equiseta, or Horse-tails, 

 present a very regular arrangement of per- 

 pendicular air-passages in the thin walls of 



their hollow stems, seen well in cross sec- 

 tions. See EQUISETACEJE. 



AIR-SACS in Plants. The genus Utri- 

 cularia, or Bladder-wort, takes its name 

 from a peculiar structure of its leaves. The 

 common species, U. vulgaris, L., often found 

 swimming just below the surface of the 

 water, in quiet streams, is provided with a 

 curious floating apparatus, formed by modi- 

 fication of portions of the feathery leaves, 

 consisting of small membranous sacs or 

 pouches, closed by a valve. The opening of 

 the pouch is somewhat funnel-shaped ; and 

 the mouth, as also the internal walls of the 

 cavity, is furnished with curious micro- 

 scopic glandular hairs. Certain of the cells 

 contain a blue- colouring-matter, distinct in 

 its nature from chlorophyll. The valve of 

 the pouch appears to be capable of opening 

 inwards only ; so that while it is turgid with 

 sap, in the vigorous periods of life, it is kept 

 closed by the pressure of the air apparently 

 secreted within the pouch ; afterwards the 

 tissue loses its tension and the air makes 

 its way out, allowing water to enter, and 

 thus putting an end to the performance of 

 the function of the air-sac. 



BIBL. Meyen. Secretions- Or q. d. Pftan- 

 zen, 1837, p. 12, t. 5. figs. 1-6; Goppert, 

 Bot. Zeit. 1847, p. 721; Benjamin, Sot. 

 Zeit. 1848; 1 et seq.; Schleiden, Princip. 

 of Botany, Engl. transl. pp. 77-279; Hen- 

 frey, Elem. Course (Masters), 1878, p. 523 ; 

 Sachs, Lehrb. d. Sot., 1874, p. 664. 



AIR-TUBES of Insects. These are horny 

 tubes found in some insects which live in 

 w^ater, as the larvae of many Diptera and 

 some water-bugs (Nepa, Ranatra}. They 

 are placed either at the first or last ab- 

 dominal segment. See NEPA, CULEX, 

 INSECTS. 



AIR-VESSELS in Insects, see TRACHEAE. 

 In plants, see SPIRAL STRUCTURES. 



ALA'RIA, Greville. A genus of Lami- 

 nariaceae (Fucoid Algse), distinguished by 

 the superficial fructification, arranged in 

 definite patches on the surface of special 

 fronds, something like the sori of Ferns. 

 The patches consist of sporanges resembling 

 the thecae of lichens, crowded together and 

 interposed between perpendicular epidermal 

 cells. The sporanges of A. esculenta are 

 described by most authors as pyriformspo/rs 

 enclosed in a perispore ; but' they perhaps 

 produce biciliated zoospores like those of 

 Laminaria. See LAMIXARIA. 



BIBL. Harvey, Brit. Mar. Alga, p. 29, 

 pi. 3 A j Greville, Alg. Brit. p. 25, pi. 4. 



