POLYCYSTIS. 



[ 020 ] 



POLYNEMA. 



1850; Microg. 18.54; Miiller, Thalass. Sf 

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

 Haeckel, Rudiolarien, 1862 ; Furlong-, Qu. 

 Mic. Jii. i. 18G1-64; Claparede et Lach- 

 niann, Inf. 434; Wallich, Tr. Mic. Soc. 

 n, s. xiii. 75 ; Thomson, Deep Sea, 98. 



POLYCYS'TIS, Kiitz.-See Clathro- 

 CYSTis. Is a Microcystis. 



POLYCYS'TIS, Leveille (Urocystis, 

 Hallier). — A genus of Ustilagiuei (Hypo- 

 devmous Fungi), including several of the 

 old species of Uredo ; P. colckici, P. parul- 

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



GINEI. 



POLYE'DPtlUM, Niig.— 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- 

 spinum (PI. o. fig. 13). 



BiBL. Rabenh. Aly. iii. 61 ; Archer, Qu. 

 Mic. Jn. 1871, 96. 



POLYEM'BRYONY.— This term is ap- 

 plied to a phenomenon occurring sometimes 

 regularly, sometimes abnormally in the de- 

 velopment of the ovules of Flowering Plants. 

 In the Angiospermous 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 Orchidacepe, 

 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 Santalaceaj. Viscitm 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 (Couiferfe and Cy- 

 cadacese), as described in the article Ovule, 

 there may be one or more (Taxns) primary 

 embryo-sacs, in which are produced several 

 corpnsada, with secondary embryo-sacs; 

 furtlier, the germinal masses of these, after 

 fertilization, produce suspeusors, which 

 branch at tlieir lower ends, and each pro- 

 duces four riidimentary embryos, all but 

 one of them vanishing during tlie ripening 

 of the seeds. Our space only admits of a 

 brief notice of these interesting phenomena, 



on which much interesting iufomiation wiU 

 be found in the works referred to below. 



BiBL. Meyen, On Impreynation and Poly- 

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

 Ann. N. H. xiii. 3G8 ; Mirbel and Spach, 

 Ann. Sc. Nat. 2. xx. 257 ; Criiger, JBot. Zcit. 

 ix. 57 ; Gelesnoff", Ann. Sc. Nat. 3. xiv. 189, 

 and the works of Hofmeister cited under 

 Ovule. 



POLYGAS'TIIICA.— According to Eh- 

 reriberg's system, the Infusoria were sub- 

 divided into the Polygastrica and the Rota- 

 toria. The so-called Polygastrica now cor- 

 respond to the Infusoria; the Rotatoria 

 forming a distinct class. 



POLYl'DES, Ag.— A genus of Cr^-pto- 

 nemiaceae (Florideous Algre), containing 

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

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

 high, consisting of repeatedly dichotomous, 

 purplish-brown, solid fibres, about 1-20" 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 surface in 

 minute moniliform rows. The fructification 

 consists of: — 1. favellce heAvmg spores, con- 

 tained in superficial wart-like bodies, com- 

 posed of colourless articulate filaments ; 2. 

 tetrahedrally divided tefrasjwres, embedded 

 in the peripheral filaments of the cortical 

 layer of the frond. Antheridia have not 

 yet been observed. 



BiBL. Harvey, Mar. Alq. 146 ; Phyc. 

 Brit. pi. 95 ; Greville, Ah/. ^Brit. pi. 11. 



POLYKRIKOS, But.-chli.— A genus of 

 Holotrichous Infusoria. P. Schicartzii, 

 marine. (Kent, Inf. 508.) 



POLYMORPIli'NA, D'Orb.— A genus 

 of Hyaline Foraminifera. luequilateral, 

 oblong or elongate, globose or compressed 

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

 ohlonya ; fig. 43, P. co7npressa) ; chambers 

 often numerous, alternate in two rows, 

 sligluly 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 apertm-es (P. Orliynii, PL 23. 

 fig. 41). 



Many species in all seas ; fossil from the 

 Trias upwards. 



JjiBL. Williamson, Pec. For. 70 ; Car- 

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

 Linn. Tr. xxvii. 107. 



POLYNE'MA. — A genus of Jlymenop- 

 terous insects. The^perfect insect is aquatic 



