POLYCOCCUS. 



[ 521 ] 



POLYEMBRYONY. 



POLYCOCCUS, Kiitz.— Probably be- 

 longs to Protococcus. 



POLYCYSTINA, Ehr.— A group of mi- 

 croscopic animal bodies, regarding the nature 

 of which little is known. They consist of 

 shells of various forms (PL 31. figs. 23-31), 

 rounded, conical, oval, radiate, star-shaped, 

 &c., often furnished with spines and other 

 processes, and sometimes constricted so as to 

 give them a jointed appearance. The shells 

 are sihceous, everywhere perforated by coarse, 

 rounded or angular foramina; and at one 

 end, sometimes at both, is a larger aperture. 



They are most abundant as fossils in the 

 rocks of Bermuda ; but have also been 

 found in the chalk and marls of Sicily, at 

 Oran in Africa, in Greece, in the tripoli of 

 Richmond in Virginia. A very few have 

 been found recent in mud at the bottom of 

 the sea, near Cuxhaven in the North Sea, 

 and near the South Pole. 



The recent shells were filled with an oHve- 

 brown organic matter. 



Forty-four genera and 282 species have 

 been described. 



They appear to have most affinity with 

 the Foraminifera, near which we have 

 provisionally placed them ; but nothing is 

 known regarding the structure of the animal. 



They form beautiful microscopic objects, 

 viewed by either reflected or transmitted 

 light. 



BiBL. Ehrenberg, Taylor's Scientific 

 Memoirs (pts. 10 & 11) and Ber. d. Berl. 

 Akad. 1846 & 1847 (Schomburgk, Ann. Nat. 

 Hist. 1847. XX. 115). 



POLYCYSTIS, Kiitz.— A genus of Pal- 

 mellacese (Confervoid Algae). This genus is 

 excluded from our synopsis of the family, 

 but since that article has been in print we 

 have found the plant, which plainly corre- 

 sponds to P. ceruginosa of Kiitzing, a form 

 separated by him from the genus Microhaloa 

 of Biasoletto, also named in his Tab. Phycol. 

 Microcystis ceruginosa. The plant in ques- 

 tion occmTcd colouring large tracts of the 

 lake in Kew Gardens, appearing to the naked 

 eye like a coarse green insoluble powder 

 suspended in the water near the surface, 

 acquhing a verdigris colom* when dried on 

 the mud. It consists of little gelatinous 

 fronds, varying in form, from 1-100 to 1-40" 

 or more in diameter, the interior of which is 

 densely crowded with separate green cells, 

 not more than 1-6000" in diameter. The 

 primary form of the gelatinous fronds appears 

 to be globular or ovate ; as they expand 

 they become hollow, forming gelatinous sacs, 



which sometimes burst laterally, but in 

 normal cases seem to expand, until, the walls 

 giving way in places, they form coarsely 

 latticed sacs of irregular form, somewhat 

 resembling on a small scale the hymenium 

 of Clathrus cancellatus. These break up 

 into irregular fragments which form new 

 starting-points for a similar development. 

 Besides this, the primary cells increase in 

 number by division into two, so that the 

 large fronds have the jelly almost as crowded 

 as the small. We suspect that certain monad- 

 Uke bodies found moving about the fi'onds 

 were zoospores, but this point is not yet 

 ascertained, neither is any true spore-forma- 

 tion. 



BiBL. Kiitz. Sp. Alg. p. 210, Tah. Phyc. 

 i. pi. 8; Meneghini, Monog. Nostoch. {Trans, 

 Turin. Acad. 2 ser. v. p. 104), 



POLYCYSTIS, LeveiUe.— A genus of 

 Ustilagines (Coniomycetous Fungi), inclu- 

 ding several of the old species of Uredo ; 

 P. colchici, P. parallela and P. ViolcB are 

 British. See Ustilagines. 



BiBL. Berk, and Broome, Ann. Nat. Hist. 

 ser. 2. V. p. 464 ; LeveiUe, Ann. des Sc. nat. 

 3 ser. V. p. 269; Tulasne, id. vii. p. 217. 



POLYEMBRYONY.— This term is ap- 

 pUed to a phaenomenon occurring sometimes 

 regularly, sometimes abnonnally in the deve- 

 lopment of the ovules of Flowering Plants. 

 In the Angiospermous plants it is usual to 

 find several germinal vesicles in the unferti- 

 lized embryo-sac (see Ovule), but ordinarily 

 only one of these becomes impregnated and 

 developed. Occasionally, however, more 

 than one commences the course of develop- 

 ment into an embryo, as in the Orchidaceae, 

 and more especially in the genus Citrus ; in 

 most cases all but one become subsequently 

 obliterated, but in the orange this is not 

 the case, and ripe seeds are met with con- 

 taining more than one embryo. We have 

 met with them in other cases. 



Another kind of polyembryony occurs in 

 the Santalaceae. Viscum has two or three 

 embrj'O-sacs ; these may all have their ger- 

 minal vesicles fertiUzed, and the development 

 of the embryos may go on to a certain point, 

 until one takes the lead and the others dis- 

 appear. 



In the Gymnospermia (Coniferae and Cy- 

 cadaceae), as described in the article Ovule, 

 there may be one or more {Taxus) primary 

 embryo-sacs, in which are produced several 

 corpuscula, with secondary embryo-sacs ; 

 further, the germinal vesicles of these, after 

 fertilization, produce suspensors, which 



