L 17 ] 



AGATE. 



sorb moisture from the atmosphere. These 

 aerial roots attach themselves to the cracked 

 bark of trees by fine hairs. The hairs con- 

 sist of cells in which a delicate spiral fibre is 

 rolled up in close convolutions. These were 

 first pointed out by Meyen, but he appears 

 to have overlooked the primary cell-wall, and 

 regarded the spiral-fibre as the sole consti- 

 tuent of the structure. The external coat of 

 the root is likewise composed of cells con- 

 taining a spiral fibre. 



BIBL. Meyen, Pflanzenphysiologie, i. p. 19. 

 t. 4. fig. 14 ; Link, Ann. Nat. Hist. ser. 2. 

 v. p. 40. 



^THALTUM, Link. A genus of Myxo- 

 gastres (Gasteromycetous Fungi). The com- 

 mon jEthalium, dE. septicum, L. (flavum, 

 Grev.), occurs frequently on tan in hot- 

 houses, where it is very injurious, 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. 



BIBL. Greville, Sc. Crypt. Flora, t.272; 

 Sowerby's Fungi, t. 399. fig. 1 (as Reticularia 

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

 and R. cerea) ; Bolton, Brit. Fungi, 1. 134 (as 

 Mucor septicus, L.). 



AGARICINI. A family of Hymenomy- 

 cetous Fungi, characterized by bearing their 

 basidiospores on thin fleshy lamellae or gills, 

 arranged vertically on the under side of a 

 stalked cap, as in the common Mushroom. 

 The basidia are elliptical or elongated cells 

 growing out from the surface of the lamellae, 

 with four slender stalk-like processes at the 

 upper end, each bearing a single spore, which 

 becomes detached when ripe. These basi- 

 diospores are observed by means of cross 

 sections of the lamellae ; the sections must 

 be very thin, and require a high power for 

 satisfactory observation. The sections keep 

 tolerably well put up in chloride of calcium, 

 and are most instructive when taken from a 

 series of specimens of different ages. See 

 AGARICUS, BASIDIOSPORES and HYME- 



NOMYCETES. 



BIBL. Berkeley on the Fructification of 

 Hymenomyc. Fungi, Ann. Nat. Hist. vol. i. 

 81 ; Leveille, Sur VHymenium des Cham- 

 pignons, Ann. des Sci. nat. 2 ser. viii. 321. 



AGARICUS, Linn. A genus of Agari- 

 cini (Hymenomycetous Fungi), of which the 

 common Mushroom, Agaricus campestris, is 

 the most familiar example. The spawn of 

 mushrooms consists of the flocculent myce- 

 lium, or vegetative structure, from which the 



fruits arise, and its nature can only be de- 

 tected by the microscope, which shows it 

 to be composed of a multitude of branched, 

 densely interwoven filaments with colourless 

 contents. 



The fructification of the Agarics consists 

 of basidiospores, arising from the sides of 

 the gills or vertical plates under the cap ; 

 they may be seen by making exceedingly 

 delicate cross-slices of the gills, and exami- 

 ning them under a high power. An eighth 

 objective is requisite to get a satisfactory 

 view. For the characters of the BASIDIO- 

 SPORES, see under that head. 



AGATE. This well-known mineral is an 

 aggregate of other mineral substances, as 

 chalcedony, jasper, amethyst, and other va- 

 rieties of quartz. It consists, chemically, 

 almost entirely of silica, coloured by metallic 

 oxides. Its interest in relation to the micro- 

 scope depends upon the supposed organic 

 remains found in it. The animal remains have 

 been especially examined by Mr. Bowerbank 

 in agates, principally moss-agates, and other 

 siliceous bodies, as jasper, the flints of the 

 chalk and greensand, &c., which he very 

 ingeniously supposes to have originated in 

 the continued attraction and solidification by 

 sponges, of silex dissolved in the water of the 

 ancient ocean; these sponges formerly ex- 

 isting at the bottom of the sea in as great 

 abundance as their recent types are now 

 found in the ocean, both in tropical and 

 temperate climates. The spicula of sponges 

 are commonly found; also very frequently 

 the fibres, sometimes in a perfect state of 

 preservation, but usually presenting the 

 appearance of having suffered to a great 

 extent from maceration and disruption of 

 their component parts previous to fossiliza- 

 tion. Generally the fibres adhere together 

 in confused masses, with here and there one 

 or two in a better state of preservation, and 

 occasionally, near the outer surface of the 

 mass, small portions of the tissue are found 

 quite perfect; in other parts all the inter- 

 mediate states between perfect preservation 

 and nearly complete decomposition may be 

 observed. The siliceous matter in which 

 these remains are imbedded, usually presents 

 a clear and frequently a crystalline aspect, 

 while the remains of the organized mass are 

 strongly tinted with colours, bright red, 

 brown and ochre-yellow prevail, but occa- 

 sionally the fibres are milk-white or bright 

 green. Sometimes the interior of the tubular 

 fibre only is filled with colouring matter, 

 whilst the sides are semipellucid or of a 



