OIDIUM. 



[ 553 ] 



ONCOBRYSA. 



lieiiii, Jahrb. i. 1 ; De Bary, A7in. Sc. N. 4 

 s^r. V. 262 ; Sachs, Bot. 281. 



OID'IUM, Link ( Acrosporium ?ir\A. Spoi-o- 

 tru'/nim, Greville ; Torula, Corda). — A sup- 

 posed geuus of Mucedines (Hj^phomycetous 

 Fungi), but very probably consisting of 

 imperfect conditions of plants of more 

 complex nature. The Oidia have recently 

 attracted great attention on account of the 

 extraordinary development of the form 

 called Oidiuin Tuckeri on the vines of Eu- 

 rope and the Atlantia islands. This, how- 

 ever, like O. leticoconiion and others, appears 

 to be only the conidiiferous mycelium of an 

 Erysiphe or some allied plant ; the parti- 

 culars of its history are given more at length 

 under Vixe-Fungus. Oidi'mn lacfis seems 

 also referable to Torula, or to the myce- 

 lium of PENiciLLitTM, O. abort if (icims , 

 Lk., is an imperfect state of Cla^iceps ; O. 

 albicavs, Robin, the fungus of Aphtha, 

 is probably referable to some other genus 

 when mature, as Achorion should perhaps 

 also be included under Penicillium. The 

 objects described as Oidia consist of delicate 

 horizontal filaments, creeping over leaves, 

 fruits, or decaying vegetable and animal 

 substances (O. lactis at the edges of sour 

 milk, O. albicans in the mouth of the hu- 

 man subject), forming an interlaced fleecy 

 coat, the horizontal filaments giving origin 

 to numerous erect, usually short, articu- 

 lated pedicels, the uppermost cells of which 

 (or several of the uppermost) expand into 

 oval bodies (conidia) which become dis- 

 articulated, and, falling upon the matrix, 

 germinate and produce new filaments (PI. 26. 

 figs. 8-11 ). 



Oiditim leucoconimn, Tuckeri, erysiplioides 

 are white ; O. aureum,fiilTvm,fructigen%im, 

 and others subsequently become coloured, 

 and these certainly belong to a different 

 categorv. 



BiBL. Berk. Hook. Br. Fl. ii. pt. 2. 349 ; 

 Ann. N. H. i. 263, vi. 438, 2 ser. vii. 178, 

 xiii. 463 ; Crypt. Bot. 300, 308; Fries, Smn. 

 Vet/. 494 ; Fresenius, Beitr. z. Mycol. H. i. 

 23, ii. 76 ; Leveille, Ann. Sc. Nat. 3 ser. xv. 

 109; Grev., Crj/pt. i^/.pl.73; Puibin, Vcr/et. 

 Par. 2nd ed. 488 ; and the Bibl. of Vine- 



FUNGTTS. 



OIKOM'ONAS, Kt.— A genus of Flagel- 

 late Infusoria. Exceedingly minute, ovate, 

 flagellum single ; a thread-like process pro- 

 truded at will from the posterior part of 

 the body, serving for attacliment. Six 

 species; in infusions, fresh- and salt-water. 

 (Kent, Inf. 250). 



OIL. — Oils of various kinds are most 

 abundantly produced by a very large num- 

 ber of plants, and occur to some extent in 

 almost all. For the microscopist, it is con- 

 venient to divide them into essciitial and 

 fixed oils. The former are special secre- 

 tions, and occur in the cells of the Glands 

 and Glandular Hairs of the epidermis of 

 those parts of plants exposed to the air and 

 light. Fixed oils are found principally in 

 the cells of tissues stiU physiologically 

 active in the nutrition of the plants, and 

 they appear in many cases to have a close 

 relation with and to form substitutes for 

 starch. Thus fixed oils occur stored up in 

 the cells of the perisperms or of the cotyle- 

 dons of certain seeds in which little or no 

 starch is produced, as in the Papaveracecs, 

 Cruciferce, Liniim, the almond, nut, &c. 

 Oil may occur also in the pulp of fruits, as 

 in the olive. 



Spores of Cryptogamic plants and Pol- 

 LEN-grains are remarkable for tlie oil they 

 exhibit in their mature condition. It ap- 

 pears to serve as an indifferent or inert form 

 of assimilated nutriment. 



Oil occurs in the cavity of cells in the 

 form of minute drops, which may be dis- 

 tinguished mostly, by the experienced mi- 

 croscopist, by simple inspection ; but it is 

 often desirable to prove the nature of the 

 globules, which may be done by removing 

 them with benzole, or, in the case of pollen, 

 by viewing them in spirit of turpentine or 

 oil of lemon. 



OI'THONA, Baird.— A genus of Cope- 

 podous Entomostraca. O. spiiiiferus ; marine. 

 (Bradv, Copep., Pai/ Soc. i. 90.) 



OLEAN'DRA, 'Cav.-A geuus of As- 

 pidieae (Polypodiaceous Ferns). 6 species j 

 tropical. (Hook. Sijn. 302.) 



OLTGOCLASE. See Rocks. 



OLPIDTUM, Braun.— Like Chytridium, 

 but without operculum or rootlet, elongated 

 into a cylindrical tube ; epi- or entophytic, 

 on freshwater Algje ; O. simidans, in the 

 living leaves of the dandelion. ( Rabeuhorst, 

 Air/, iii. 282.) 



OMPHALOPEL'TA, Ehr.— A genus of 

 fossil Diatomaceoe which agrees with Acti- 

 noptyclins. 



O. areolata (PI. 18. fig. 53). 



Bibl. Ehr. Ber. Berl. Ak. 1844, 263; 

 Kiitz. Sp. Aly. 132; Grev. Mic. Tr. 1866, 

 122. 



ONCOBRY'SA, Agardh {Hydrococcus, 

 Kiitz.). — A genus of Palmellacere. Fronds 

 minute, hardish ; cells in rows, the super- 



