=AIR.= Pure atmospheric air is composed of nitrogen, oxygen, and a very small quantity of carbonic acid gas, all of which are essential to the growth of plants. Air-giving is a term used by gardeners to lessen the temperature of a greenhouse, or to equalise it with that outside. _See_ =Ventilation=.
=AIRA= (from _aira_, applied by the Greeks to _Lolium temulentum_). Hair Grass. ORD. _Gramineæ_. Chiefly hardy grasses, of agricultural value. Panicle loose; spikelet compressed, with two perfect flowers, and sometimes a neuter. Of easy culture, in ordinary garden soil. Sow seeds in spring.
=A. flexuosa= (waved).* The Waved Hair Grass. _fl._ shining brown; panicle erect, spreading, with waved angular branches and flower-stalks. _l._ short. Stem upwards of 1ft. high, erect, smooth. England. A very pretty and graceful perennial.
=A. pulchella= (pretty).* _fl._ panicles loose, very delicate and graceful. _l._ very short. _h._ 6in. to 8in. South Europe. An elegant plant, with tufted filiform stems. One of the best of dwarf-growing ornamental grasses. See Fig. 54.
=AIR-PLANT.= _See_ =Aerides=, also =Epiphytes=.
=AITONIA= (in honour of W. Aiton, once Head Gardener at Kew). ORD. _Meliaceæ_. A small and rather interesting greenhouse evergreen shrub from the Cape of Good Hope, and thriving well in an equal mixture of sandy loam and peat. Young cuttings will root in sand, under a bell glass, with bottom heat. The cuttings must not be put in very close together, and the glass should be wiped frequently, as they are apt to damp off.
=A. capensis= (Cape). _fl._ pink; petals four, shorter than the projecting stamens. July. _h._ 2ft. 1777.
=AIZOON= (from _aei_, always, and _zoos_, alive; tenacious of life). ORD. _Portulacaceæ_. Greenhouse annuals, biennials, or evergreen shrubs. Flowers apetalous; calyx five-cleft, coloured on the inner surface. The undermentioned species is the only one worth growing. It requires no shade, a dry atmosphere, and light sandy soil. Propagated by seeds and cuttings.
=A. sarmentosum= (sarmentose). _fl._ greenish, sessile. Summer. _l._ opposite, linear-filiform, rather connate, glabrous; branches rather villous, three-flowered at the apex, the two lateral flowers are bracteated, and spring from the sides of the middle one. Sub-shrub, erect, diffuse, glabrous, branched. South Africa, 1862.