^TEA. 



[ 20 ] 



AGATE. 



Araceas. The surface of these aerial roots 

 is clothed by a peculiar tissue, formed of 

 cells containing a delicate spiral fibre upon 

 the wall (PL 4S. fig. G). The strata of spiral- 

 fibrous cells are sometimes numerous, and 

 they cover up the true epidermis of the 

 root. The growing points of such roots are 

 green ; but the spiral-librous cells soon come 

 to contain nothing but air, and then assume 

 a silvery-white colour. 



^TEA, Lamx. See Anguinaria, 



^THA'LIU -M, Link. A genus of Myxo- 

 mycetes. The common ^^thalium^ AL. sep- 

 tictim, L. ijlavum, Grev.), occurs frequently 

 on tan in hot-house^, where it is very inju- 

 rious, from the rapidity of its growth and the 

 abundance of its spores. The ordinary form 

 is yellow; but violet and reddish-brown 

 varieties have been met with. It grows 

 also on mosses in woods. Other species of 

 JEihalmm have been found growing upon 

 iron, lead, or other mineral substances, 

 sometimes a few hours only after they have 

 been heated, so that the appearance seems 

 quite marvellous. The cream-like matter, 

 of which the part answering to the myce- 

 lium or allied production in other Fungi 

 consists, exhibits Amwha-\\k'? movements. 



BiBL. GreviUe, Crypt, Flora, t. 272; 

 Sowerby's i^«?25^j, t. 399. fig. 1 (as liettcularia 

 hortensis, Bull.), figs. 3 & 4 (as R. carnosa 

 and H. cere(i) ; Bolton, Brit. Fungi, t. 134 

 (as Mucor sepfiats, L.) ; Berkeley, Cryjtt. 

 Bot. pp. 23G, 339. 



AGARICIM.— A family of Basidiomy- 

 cetous Fungi, belonging to the tribe Ilyme- 

 nomycetes, comprising a great portion of 

 the more important esculent species, cha- 

 racterized by an inferior hymenium spread 

 over distinct gill-like processes, which are 

 often easily divisible into two plates. In a 

 few species the interstices of the gills are 

 traversed by veins so as to produce the sem- 

 blance of pores. Amongst the more obscure 

 species of the vast genus Ayaricus, the hy- 

 menium is at first superior, but finally be- 

 comes inferior by the turning over of the 

 pileus, which is attached at one point only, 

 or by a very short stem. 



Tiie hymenium is composed of vertical 

 cells, called hasidia by L6veill^, sporophores 

 by Berkeley. These bodies are elliptical or 

 elongated cells growing out from the surface 

 of the lanielhe, with four slender stalk-like 

 processes at the upper end, each bearing a 

 single spore, which becomes detached when 

 ripe. These basidiospores are observed by 

 means of cross sections of the lamelUe ; the 



sections must be very thin, and require a 

 high power for satisfiictory observation. 

 The sections keep tolerably well put up in 

 chloride of calcium or glj'cerine, and are 

 luost instructive when taken from a series 

 of specimens of ditFerent ages. See Aga- 

 Eicus, Basidiospores, and Hymenomy- 

 CETES. The bodies called cystidia or ^w//j- 

 fiaria, are globular or oval cells, found asso- 

 ciated' with the basidia, containing granular 

 matter exhibiting molecular motion Avhen 

 discharged. These organs have been sup- 

 posed to represent antheridia, but are more 

 probably paraphyses or abortive basidia. 



BiBL. Berkeley on the Fructif. of llyme- 

 nomye. Fungi, Ann. N. Jlid.i.Sl ; Leveille, 

 Siir l^ Hymenium des Champignons, Ann. S'c, 

 Nat. 2 ser. viii. 321 ; Hoffman, Bot. Zeit. 

 xiv. p. 137 ; Cooke, Illustr. Brit. Fungi, 

 1881. 



AGARICUS, Linn.— A genus of Agari- 

 cini (Hymenomycetous Fungi ; one of the 

 largest genera in the Vegetable Kingdom, 

 comprising not only a multitude of European 

 species, but many from tropical climates. 

 Ag. catnpestris is the common Mushroom. 



See Hymenomycetes. 



AGATE. — The term agate is specially 

 applied by geologists to the concretions 

 and geodes of chalcedony, formed in the 

 steam-holes and the fissures of volcanic 

 rocks. They consist of crystallized and 

 crystalline varieties of silica, coloured with 

 metallic oxides. Successive and concentric 

 layers of chalcedony, coating the insides of 

 the cavities, have formed solid or hollow 

 nodules, the innermost layers being often 

 colourless or amethystine quartz. In the 

 chalcedony no definite crystallization can be 

 seen, only an obscure fibrous structure at 

 right angles to the planes of deposit, which 

 latter are usually Imnpy or mammillary, and 

 often render the mass flaky. In some 

 specimens this fibrous structure becomes 

 more evident in successive layers — minute, 

 close-set, crystalline prisms being visible on 

 fracture ; and these may be succeeded by 

 regular quartz-crystals. Those paler vari- 

 eties of quartz, which consist of concentric 

 layers of radiately grouped cr^'stalline 

 needles, frequently polarize light very 

 beautifully. From the unequal resistance 

 of some of the layers to hydrochloric acid, 

 cut agates may be prepared' to take printer's 

 ink and to give impressions on paper. The 

 relatively open prismatic structure of some 

 layers allows agates to be partially coloiu-ed 

 by the absorption and carbonization of oil 



