POLYCYSTIS. 



[ 020 ] 



POLYNEMA. 



1850; Microg. 1854; Miiller, Thalass. # 

 Polycyst., Abh. Ak. Wiss. Berlin, 1858; 

 Haeckel, Radiolarien, 1802 ; Furlong, Qu. 

 Mic. Jn. i. 1801-64; Claparede et Luch- 

 mann, Inf. 434; Wallich, Tr. Mic. Sac. 

 n. s. xiii. 7o ; Thomson, Deep Sea, 98. 



POLYCYSTIS, Kutz.-See CLATHRO- 

 CYSTIS. Is a Microcystis. 



POL YC YS'TIS, Le veillS ( Uroci/stis, 

 Hallier). A genus of Ustilagiuei (Hypo- 

 dermous Fungi), including several of the 

 old species of Uredo ; P. colchici, P. paral- 

 lela, and P. violce are British. See USTILA- 

 GINEI. 



POLYE'DRIUM, Nag. A genus of 

 Unicellular Algae. 



Char. Cells single, 3-4-8-angular, the 

 angles more or less produced. Several 

 species, in freshwater pools. P. longi- 

 sjnnum (PI. 3. fig. 13). 



BIBL. Rabenh. Aly. iii. 01 ; Archer, Qu. 

 Mic. Jn. 1871, 90. 



POLYEM'BHYONY. This term is ap- 

 plied to a phenomenon occurring sometimes 

 jegularly, sometimes abnormally in the de- 

 velopment of the ovules of Flowering Plants. 

 In the Angiosperrnous plants it is usual to 

 find several germinal masses 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 the embryo, as in the Orchidacese, 

 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 Santalacese. Viscum has two or three 

 embryo-sacs ; these may all have their ger- 

 minal masses fertilized, and the develop- 

 ment of the embryos may go on to a certain 

 point, until one takes the lead and the 

 others disappear. 



In the Gymnospermia (Coniferae and Cy- 

 cadaceas), as described in the article OVULE, 

 there may be one or more (Taxus) primary 

 embryo-sacs, in which are produced several 

 corpuscida, with secondary embryo-sacs; 

 further, the germinal masses of these, after 

 fertilization, produce suspensors, which 

 branch at their lower ends, and each pro- 

 duces four rudimentary embryos, all but 

 one of them vanishing during the ripening 

 of the seeds. Our space only admits of a 

 brief notice of these interesting phenomena, 



on which much interesting information will 

 be found in the works referred to below. 



BIBL. Meyen, On Impregnation and Poly- 

 embryomj, Taylor's Sc. Mem. iii, 1 ; Brown, 

 Ann. N. H. xiii. 368 ; Mirbel and Spach, 

 Ann. Sc. Nat. 2. xx. 257 ; Criiger, But. Zcif. 

 ix. 57 ; Gelesuoff, Ann. Sc. Nat. 3. xiv. 189, 

 and the works of Hofmeister cited under 

 OVULE. 



POL Y^GAS'TRIC A. According to Eh- 

 renberg's system, the Infusoria were sub- 

 divided into the Polygastrica and the Rola- 

 toria. The so-called Polygastrica now cor- 

 respond to the Infusoria; the Rotatoria 

 forming a distinct class. 



POLYI'DES, Ag. A genus of Crypto- 

 nenriaceae (Florideous Algre), containing 

 one British species, P. rotundus (PI. 4. 

 fig. 3), having a branched frond 4 to 6" 

 high, consisting of repeatedly dichotomous, 

 purplish-brown, solid fibres, about l-'JO" in 

 diameter. The fibres present a central layer 

 of longitudinally arranged filamentous cells, 

 and a cortical layer of perpendicular, dicho- 

 tomous filaments, formed of elliptical cells 

 internally, terminating at the surfa- -e in 

 minute moniliform rows. The fructification 

 consists of: 1. favellce bearing spores, con- 

 tained in superficial wart-like bodies, com- 

 posed of colourless articulate filaments ; 2. 

 tetrahedrally divided tetraspores, embedded 

 in the peripheral filaments of the cortical 

 layer of the frond. Antheridia have not 

 yet been observed. 



BIBL. Harvey, Mar. Alg. 146; Pliyc. 

 Brit. pi. 95 ; Greville, Al<j. Brit. pi. 11. 



POLYKRIKOS, Butschli. A genus of 

 Holotrichous Infusoria. P. iSchwurtzii, 

 marine. (Kent, Inf. 508.) 



POLYMORPHPNA, D'Orb. A genua 

 of Hyaline Foraminifera. Inequilateral, 

 oblong or elongate, globose or compressed 

 (PI. 23. fig. 40, P. communis; fig. 42, P. 

 oblonga ; fig. 43, P. compressa] ; chambers 

 often numerous, alternate in two rows, 

 slightly embracing, but always more so on 

 one side than the other ; orifice round, at 

 the summit of the last chambers, radiate. 

 Sometimes the later chambers have branch- 

 ing, tubular apertures (P. Orbignii. PI. 23. 

 fig. 41). 



Many species in all seas ; fossil from the 

 Trias upwards. 



BIBL. Williamson, JRec. For. 70; Car- 

 penter, For. 166 ; Brady, Parker, and Jones, 

 Linn. Tr. xxvii. 197. 



POLYNE'MA. A genus of Hymenop- 

 terous insects. The perfect insect is aquatic 



