OIDIUM. 



[ 553 ] 



ONCOBRYSA. 



heim, Jahrb. i. 1 ; De Bary, Ann. So. N. 4 

 &\\ v. 262 ; Sachs, Bot. 281. 



OID'IUM, Link (Acrosporium and Sporo- 

 tn'c/uim, Greville ; Tontla, Corda). A sup- 

 posed genus of Mucedines (Hyphomycetous 

 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 Oidium Tuckeri on the vines of Eu- 

 rope and the Atlantic islands. This, how- 

 ever, like O. leucoconium 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-FuxGUS. Oidium lactis seems 

 also referable to Torula, or to the myce- 

 lium of PENICILLIUM. O. abortifaciens, 

 Lk., is an imperfect state of CLAVICEPS ; O. 

 albicans, 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 

 inilk, 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 (PL 26. 

 figs. 8-11). 



Oidium leucoconium, Tuckeri, erysiphoides 

 are white ; O. aureum^fidrum^fructigenum^ 

 and others subsequently become coloured, 

 and these certainly belong to a different 

 category. 



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, Sum. 

 Vey. 494 ; Fresenius, Beitr. z. Mycol. H. i. 

 23, ii. 76 ; Le>eille, Ann. Sc. Nat. 3 ser. xv. 

 109; Grev., Crypt. _F/.pl.73; Robin, Veget. 

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



FUNGFS. 



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 attachment. 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 essential 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 still 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 Papaveracea, 

 Cruciferce, Linum, 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 the 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. spiniferus ; marine. 

 (Brady, Copep., Hay Soc. i. 90.) 



OLEAN'DRA, Cav. A genus of As- 

 pidiese (Polypodiaceous Ferns). 6 species ; 

 tropical." (Hook. Syn. 302.) 



OL'IGOCLASE. See ROCKS. 

 " OLPID'IUM, Braun. Like Chytridium, 

 but without operculum or rootlet, elongated 

 into a cylindrical tube ; epi- or entophytic, 

 on freshwater Algae ; O. simulans, in the 

 living leaves of the dandelion. (Rabenhorst. 

 Alff. iii. 282.) 



OMPHALOPEL'TA, Ehr. A genus of 

 fossil Diatom aceae which agrees with Acti- 

 noptychus. 



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



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

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

 122. 



ONCOBRY'SA, Agardh (Hydrococcus, 

 Kiitz.). A genus of Palmellaceae. Fronds 

 minute, hardish ; cells in rows, the super- 



