NULLIPORES. 



[ 467 ] 



(ECISTES. 



They are best seen in very young cells in 

 all cases ; in nascent tissues they almost or 

 quite fill the cavity of the young cells. As 

 the cells grow older, their history differs in 

 different cases. Sometimes they persist 

 until the decay of the organ in which they 

 exist. This happens very generally in the 

 cells of the flowers, stems, &c. of Monoco- 

 tyledons; not unfrequently, in stems and 

 leaves they become converted into starch or 

 chlorophyll-granules. In other cases they 

 have a more definite purpose, for in the 

 vesicles in which are formed the sperma- 

 TozoiDS of Ferns, Mosses, Hepaticae, 

 Characece, &c., these structures appear to 

 be produced by a metamorphosis of the 

 nuclei. 



In examining supposed nuclei of plants, 

 especially those of lower cellular organization, 

 tincture of iodine should always be applied, 

 to distinguish starch-granules, &c. fi'om true 

 nuclei, which are always coloured deep yel- 

 low or brownish by that reagent, besides 

 being coagulated, contracted and thereby ren- 

 dered more distinct. 



The nuclei of plants require much more 

 investigation. 



BiBL. R. Brown, on Orchidacecs, Phil. 

 Mag. Dec. 1831 ; Schleiden, Phytogenesis, 

 Midler^ s Archiv, 1838, transl. in Sc. Memoirs, 

 ii. p. 281, Grundzilge, 3 ed. {Principles, p. 

 568) ; Nageli, Zeitschr. fur Wiss. Bot. 

 (transl. in Ray Soc. Vols. 1845 & 1849) ; 

 Mohl, Pjianzen-zelle {Vegetable Cell), pp. 

 36 & 51 ; Hofmeister, Entsteh. d. Embryo, 

 Leipsic, 1849. p. 7; Al. Braun, Verjungung 

 {Ray Soc. Vol. 1853. p. 175). 



NULLIPORES. See Corallinace^. 



NYMPHyEACEiE. See Hairs (p. 313). 



O. 



OAT, Avena sativa (Nat. Order Grami- 

 nacese. Flowering Plants). — The form of the 

 starch-corpuscles of the oat is very unlike 

 that of the other common corn plants, con- 

 sisting of numerous small polygonal grains 

 grouped together in roundish or oval masses 

 (PI. 36. fig. 10). See Starch. 



OCHLOCH^TE, Thwaites.— A genus of 

 Chaetophoraceae (Confervoid Algae), consist- 

 mg of minute plants growing epiphytically 

 on leaves of grasses, &c. O. hystrix occurs 

 both in brackish and freshwater ditches. 

 The minute, dot-like, discoid frond is formed 

 of radiating branched filaments composed of 

 cells, each bearing a very long tubulai* fila- 



ment on its back. Fructification unknomi. 

 We suspect this plant is closely connected 

 with the lax forms of CoLEOCHiETE. 



BiBL. Harvev, Brit. Mar. Alg. p. 211. 

 pi. 25 E, Phyc. Brit. pi. 226. 



ODONTELLA, Ag.— This genus of Dia- 

 tomaceae is united with Biddulphia, Bid- 

 dulphia {Odontella) aurita undergoing spon- 

 taneous division, PI. 14. fig. 9. 



ODONTHALIA, Lyngb.— A genus of 

 Rhodomelaceae (Florideous Algae) containing 

 one British species, O. dentata, which has 

 an irregularly bipinnatifid frond, 3 to 12" 

 long, the main axis and lobes being about 

 1-4" wide throughout; the colour is deep 

 wine-red, darker when dried. The frond 

 bears marginal, stalked, ostiolate, ovate ce- 

 ramidia with spores; lanceolate stichidia, 

 in which are contained two rows of ternate 

 tetraspores; and antheridia. 



BiBL. Harv. Brit. Alg. p. 77- pi. 11 A, 

 Phyc. Brit. pi. 34 ; Greville, Alg. Br. pi. 13; 

 Kiitzing, Phyc. generalis, p. 448. 



ODONTIDIUM, Kg.— A genus of Dia- 

 tomaceae. 



Char. Frustules quadrangular, united to 

 form an elongated biconvex filament ; linear 

 in front view ; side view (valves) with trans- 

 verse striae (visible under ordinary ilium.). 

 Aquatic and marine. 



Differs from Denticula in the elongated 

 filament, which sometimes, however, con- 

 sists of only three or four frustules ! 



Kiitzing describes fifteen species, two 

 doubtful. 



0. turgidulum (PI. 13. fig. 14; a, front 

 view; b, side view). Valves lanceolate, ob- 

 tusish ; striae on each valve six. Aquatic ; 

 length of frustules 1-1720 to 1-570". 



BiBL. Kiitzing, Bacill. p. 44, Sj). Alg. 

 p. 12. 



' ODONTODISCUS, Ehr.— A genus of 

 Diatomaceae. 



Char. Frustules single, lenticular; valves 

 circular, alike, without nodules or apertures, 

 not areolar (under ordin. ilium.), but covered 

 with puncta either arranged in radiating 

 rows, or in excentrically curved lines, and 

 with erect marginal teeth. 



The puncta are surely the ordinaiy de- 

 pressions imperfectly examined. 



Three species. Fossil and in guano. 

 Diameter from 1-860 to 1-240". 



BiBL. Ehrenberg, Ber. d. Berl. Akad. 

 1844. p. 73 ; Kiitzing, Sp. Alg. p. 129. 



(ECISTES, Ehr.— A genus of Rotatoria, 

 of the family CEcistina. 



Char. Single ; rotatory organ single, with 



2h2 



