AIR-BLADDER. 



ALARIA. 



PL 37. fig. 20 ; in the delicate cavities of a 

 hair in PL 22. fig. 1 ; and the lower part of 

 the same figure represents a portion from 

 which the air has been displaced by liquid. 



AIR-BLADDER of Fishes. See SWIM- 

 MING BLADDER. 

 AIR-BUBBLES. See AIR. 

 AIR-CELLS of animals. These are dila- 

 tations or expansions of the air-passages; but 

 a distinction must be made between them 

 and the lungs, which might be regarded also 

 as air-cells. See LUNGS. 



The proper air-cells or air-sacs, as met 

 with in birds, are membranous cavities com- 

 municating' with the lungs and distributed 

 through the chest and abdomen. These 

 air-sacs, or prolongations of them, 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 TRACHEAE. 

 Their obvious use is either to diminish 

 the specific gravity of the body, or to act as 

 reservoirs of air during the impeded respira- 

 tion connected with flight. 



BIBL. Siebold and Stannius, Lehrb. d. 

 vergleich. Anat.; Owen, Hunterian Lectures; 

 Carpenter, Man. of Compar. Anat. 



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. 

 In the Nymph&acece (Water-lily family) the 

 large air-passages in the floating leaves and 

 the stem have peculiarly developed star-like 

 cells projecting fully into these cavities; 

 these cells are filled with a granular substance 

 very unlike the contents of the large cells of 

 the general parenchyma of the leaf. Their 

 nature and office are yetunknown. They occur 

 in the common water-lilies andintheFcora. 

 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 sections. 

 See EQUISETACE.E. 



AIR-SACS in plants. The genus 

 Utricularia, or Bladder-wort, takes its name 

 from a peculiar structure of its leaves. The 

 common species, U. vulgaris, L., may be 

 often found swimming just below the surface 

 of the water, in quiet streams, and sending 

 up a little yellow blossom above the water 

 in autumn. It is provided with a curious 

 floating apparatus, formed by modification 

 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, are 

 furnished with curious microscopic 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 vi- 

 gorous 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, allow- 

 ing water to enter, and thus putting an end 

 to the performance of the function of the 

 air-sac. 



BIBL. Meyen, Secretions-organe der 

 Pflanzen, Berlin, 1837, p. 12. t. v. figs. 1-6 ; 

 Goppert, Botanische Zeitung, 1847, p. 721 ; 

 Benjamin, Bot.Zeit. 1848, 1 et seq.; Schlei- 

 den, Principles of Botany, English trans- 

 lation, pp. 77-2/9. 



AIR-TUBES of Insects. These are horny 

 tubes found in some insects which live in 

 water, as the larvae of many Diptera and 

 some water-bugs (Nepa, Ranatra). They 

 are placed either at the first or last abdominal 

 segment. See NEPA, CULEX, INSECTS. 



AIR-VESSELS in Insects, see TRACHEAE. 

 In plants, see SPIRAL VESSELS. 



ALARIA, Greville. A genus of Fucoid- 

 eous sea-weeds belonging to the family 

 Laminariaceae, distinguished by their superfi- 

 cial fructification, in Alaria arranged in 

 definite patches on the surface of special 

 fronds, something like the sori of Ferns. The 

 patches consist of sporaiiges resembling the 

 thecae of lichens, crowded together and inter- 

 posed between perpendicular epidermal cells. 

 The sporanges are described by most authors 

 as pyriform spores enclosed in a perispore, 

 but they are probably oosporangia producing 

 biciliated zoospores like those of Laminaria. 

 See LAMINARIA. 



One British species is known, a common 

 sea-weed having a large flat and narrow leaf- 

 like frond, with a thick midrib, a prolonga- 



