DICKSONIA. 



[ 206 ] 



DICRANACEvE. 



Char. Frustules resembling those of Ach- 

 nanthes, irregularly scattered through a flat 

 undulate frond or subgelatinous layer, nar- 

 rowed at the base so as to appear substipi- 

 tate. 



D. ulvacea (PL 14. fig. 16; a, frond, nat. 

 size ; b, portion magnified ; c, prepared 

 frustule, front view ; d, valve). Stipes very 

 short, capillary ; frond oblong, irregularly 

 lobed or crenulate; frustules (front view) 

 oblong, obtuse-angled, truncate at the ends ; 

 valves narrowly linear ; length of frond 1 to 

 H"; of frustules, 1-1000 to 1-720"; marine. 



Frond very pale purplish white. Recent 

 frustules with a round colourless spot at each 

 of the four angles (in the front view). Found 

 in shallow pools between high and low-water 

 mark. 



D. Danseii. Frond indefinite, mammillate ; 

 valves oval. The frustules of this species 

 appear also to resemble those of Achnanthes ; 

 but the form and structure of the frond do 

 not correspond with the characters of the 

 genus Dickieia, hence either the generic 

 characters must be altered, or this species 

 placed in a new genus. 



BIBL. Berkeley and Ralfs, Ann. Nat. Hist. 

 1844, xiv. p. 328; Kiitzing, Bacitt. p. 119, 

 Sp. Alg. p. 109 ; Thwaites, Ann. Nat. Hist. 

 1848.1. p. 171. 



DICKSONIA, L'Heritier. A genus of 

 Cyathseous Ferns. Exotic. 



DICLADIA, Ehr. A genus of Diatoma- 

 eeae. 



Char. Frustules single ; valves unequal, 

 one turgid and simple, the other two-horned, 

 the horns sometimes branched. Marine and 

 fossil. 



Closely allied to Rhizoselenia. 



Frustules siliceous, bivalve ; minute struc- 

 ture of valves undetermined. 



Four species, none British. 



BIBL. Ehrenb. Ber. d. Berl Akad. 1844. 

 p. 73 ; Kiitzing, Sp. Alg. p. 24. 



DICOTYLEDONS. One of the two 

 great divisions of the Angiospermous Flow- 

 ering Plants, synonymous with the Exogens 

 of Decandolle, and opposed to Monocotyle- 

 dons, the name being derived from the con- 

 dition of the embryo prevailing throughout 

 the vast majority of plants included in this 

 assemblage. As in all other natural groups, 

 instances occur wherein the particular cha- 

 racter from which the name is derived, the 

 presence of a pair of cotyledons in the 

 embryo, is absent, as in Orobanche, &c. (like 

 the Orchidacese and other plants among the 

 Monocotyledons), but in these cases the 



plants agree with Dicotyledons in general in 

 all the rest of the prominent characters, such 

 as the structure of stem, leaves, plan of 

 flower, &c. See VEGETABLE KINGDOM 

 and SEED. 



DICRANACE.E. A family of Apocar- 

 pous operculate Mosses, branching by inno- 

 vations, or with the tops of the fertile branches 

 several times divided. Leaves lanceolate or 

 subulate, channeled-concave, with a nerve 

 mostly dilated and flattened, rarely slender, 

 scarcely cylindrical. The cells prosenchyma- 

 tous, often mingled with parenchymatous, 

 rarely papillose, mostly empty, often thick- 

 ened upwards, thereby rounded or elliptical ; 

 the basilar cells arranged in a curved manner 

 at the margins of the leaves, distinctly 

 diverse ; parenchymatous, lax, thick, large, 

 flat or with a more or less thick and patelli- 

 form front, delicate or robust, hyaline, fuscous, 

 brown or purple, ultimately marcescent, 

 mostly very conspicuous (alar cells). Cap- 

 sule oval or cylindrical, arched or straight, 

 apophysate or strumose at the base, with a 

 subulate operculum. Peristome, if present, 

 purple, teeth trabeculate. 



British Genera. 



I. BLINDIA. Calyptra dimidiate, hood- 

 shaped, peristome wanting or simple, then of 

 sixteen equidistant, lanceolate, distantly arti- 

 culated, smooth, slender teeth, slightly tra- 

 beculate within, purple, cartilaginous. 

 Capsule exannulate. 



II. DICRANUM. Calyptra dimidiate. Pe- 

 ristome simple, teeth connate at the base into 

 a more or less emergent membrane, or equi- 

 distant and arising below the orifice of the 

 capsule, split more or less deeply, even in 

 some cases to the base, into two or rarely 

 more free arms, purple below, traberculate 

 nodose above (figs. 172 & 173). 



Fig. 172. 



Fig. 173. 



Dicranum palustre. 



Fig. 172. Mouth of the capsule with the peristome 

 everted. Magnified 40 diameters. 



Fig. 173. Portion of the peristome. Magnified 100 

 diameters. 



