OIL. 



[ 470 ] 



ONYGENEI. 



probably referable to some other genus when 

 mature, as Achorion should perhaps also 

 be included under Puccinia. The objects 

 described as Oidia consist of delicate hori- 

 zontal filaments, creeping over leaves, fruits 

 or decaying vegetable and animal substances 

 (0. lactis at the edges of sour milk, 0. albi- 

 cans in the mouth of the human subject), 

 forming an interlaced fleecy coat, the hori- 

 zontal filaments giving origin to numerous 

 erect (usually short), articulated pedicels, 

 the uppermost cells of which (or several of 

 the uppermost) become expanded into oval 

 bodies (conidia) which become disarticulated, 

 and falling upon the matrix, germinate and 

 produce new filaments (PL 20. figs. 8, 9). 



Oidium leucoconium, Tuckeri, erysiphoides 

 are white; 0. aureum, fulvum,fructigenum, 

 and others subsequently become coloured. 



As we do not regard them as independent 

 organisms, it seems unnecessary to give the 

 characters of the supposed species. 



BIBL. Berk. Hook. Brit. Fl. ii. pt. 2. p. 

 349; Ann. Nat. Hist. i. p. 263, vi. p. 438, 



2 ser. vii. p. 178, xiii. p. 463; Fries, Summa 

 Veg.494; Fresenius, Beitr.z.Mycologie,T:Left 

 i. p. 23, ii. p. 76 ; Leveille, Ann. d. Sc. nat. 



3 ser. xv. p. 109 ; Grev. Sc. Crypt. Fl. pi. 

 73 ; Ch. Robin, Vegetaux Parasites, 2nd ed. 

 p. 488; and the Bibl. of VINE FUNGUS. 



OIL. Oils of various kinds are most 

 abundantly produced by a very large number 

 of plants, and occur to some extent in almost 

 all. For the microscopist, it is convenient 

 to divide them into essential and fixed oils. 

 The former are special secretions, 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 nu- 

 trition 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 peri- 

 sperms or of the cotyledons of certain seeds 

 in which little or no starch is produced, as 

 in the Papaveracece, Crucifera, 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 distin- 

 guished mostly, by the experienced micro- 



scopist, by simple inspection ; but it is often 

 desirable to prove the nature of the globules, 

 which may be done by removing them with 

 aether, or, in the case of pollen, by viewing 

 them in spirit of turpentine or oil of lemons. 

 Potash does not act readily upon oil-globules 

 in the cells of plants. 



In certain cases it is convenient to view 

 objects in oil instead of water, in order to 

 render them more transparent ; for this pur- 

 pose oil of lemon is conveniently employed. 



OMPHALOPELTA, Ehr. A genus of 

 fossil Diatomaceae. 



Char. Agrees with Actinoptychus, but the 

 upper part of the margin of the valves has a 

 few opposite erect spines. 



Four species. Fossil and marine. 



BIBL. Ehrenberg, Ber. d. Berl. Akad. 



1844. p. 263; Kiitzing, Sp. Ala. p. 132. 

 ONCOSPHENIA, Ehr. A genus of Dia- 

 tomaceae. 



Char. Frustules single, cuneate in front 

 view ; valves equal, uncinate at the apices ; 

 neither vittae nor nodules present. 



0. carpathica. Valves cuneate, laxly stri- 

 ated (ord. ilium.), one end turgid, rounded, 

 and straight, the other attenuate and unci- 

 nate; aquatic; diameter 1-790". 



BIBL. Ehrenberg, Ber. d. Berl. Akad. 



1845. p. 72; Kiitzing, Sp. Alg. p. 11. 

 ONION, Allium Cepa (Flowering Plants, 



Nat. Ord. Liliaceae). The young bulb of the 

 onion offers a very good and cheap subject 

 for the investigation of the development of 

 spiral vessels, to those who do not object to 

 its odour ; other bulbs will do equally well. 

 In the cells of the base of the bulb occur 

 very elegant groups of prismatic crystals 

 (see RAPHIDES). 



ONOCLEA, Linn. A genus of Cystopte- 

 rideae (Polypoda3ous Ferns). Indusium very 

 thin, membranous and reticulated. The fer- 

 tile pinnae are usually so rolled up as to look 

 like little berries seated on a spike, filled 

 with sporangia. Exotic. 



ONYGENEI. A family of Ascomycetous 

 Fungi, containing a few inconspicuous plants 

 growing upon the feathers of dead birds, or 

 upon cast-off horse-shoes. The flocculent 

 spreading mycelium usually produces on its 

 surface little white stalk-like bodies crowned 

 by a globular perithecium. At first erect and 

 thick, these supports become more slender 

 as they elongate, and seem to bend under 

 the weight of the light perithecium (fig. 536). 

 In some species the perithecium is sessile. 

 The perithecium is filled with branching 

 filaments, arising from the walls of its inter- 



