LAURENCIACE^E. 



[ 455 ] 



LEAVES. 



tetraspores are tetralieclrally divided. The 

 autlieridia ave thus described (in L. tenuis- 

 sima) by Thuret : on the smaller branches, 

 similar to those which bear the ceramidia 

 on other indi\ iduals, occur grej-ish convo- 

 luted plates of cellular tissue, of irregular 

 form, bordered by a line of roundish cells, 

 containing generally a yellow liquid. Hya- 

 line cells containing antlierozoids are im- 

 planted vertically on these plates, clothing 

 both surfaces. The antheridium has a sort 

 of pedicel formed of an ovoid cell, which 

 also bears a dichotoraous hair, like those 

 common over the branches of this plant. 

 The autherozoids are elongated-ovoid, a 

 little constricted at one extremity, length 

 about 3-5000". Derbes and Solier have 

 obsei'ved them on L. pinnatijida and other 

 species. 



BiBL. Plarvey, Mar. Ah/. 97, pi. 12 C ; 

 Phi/c. Brif. pi. 5o ; Grev. Ahj. Br. 108, pi. 

 14 ; Derbes and Solier, Ann. Sc. Kat. 3 ser. 

 xiv. 276, pi. 37 ; Thuret, ib. xvi. 65, pi. 7, 

 lb. ser. 4. iii. 19. 



LAURENClA'CE.E.— A family of Flo- 

 rideae. Rose-red or purple sea-weeds with 

 a cylindrical or compres.-ed, rarely flat, li- 

 neal", narrow, areolated, inarticulate, or con- 

 stricted and chambered, branching frond 

 composed of polygonal cells. Fructification : 

 1, conceptacles {ceramidia) external, ovate, 

 furnished with a terminal pore, and con- 

 taining a tuft of pear-shaped spores ; 2, te- 

 trasporcs immersed in the branches and 

 ramidi, scattered without order through the 

 surface cells ; 3, anfkeridia. 



British genera : 



Bonnemaisonia. Frond solid, filiform, 

 rose-red, much branched; branchesmargined 

 with subulate distichous cilia. 



Laiirencia. Frond solid, cylindrical or 

 flattened, purplish or yellowish, pinnatifid, 

 ramuli blunt. 



Chrysimenia. Frond hoUow, filled with 

 mucus, neither constricted nor chambered. 



Chylocladia. Brancheshollow, with mucus, 

 constricted at intervals. 



LEA'IA, Jones. — A fossil Entomostracan 

 Bivalve, of unknown alliance, probably a 

 Phyllopod. Valves oblong; marked with 

 two obliquely transverse, divergent ridges, 

 concentric lines of growth, and intermediate 

 reticulation. Known in the Coal-measures 

 of Britain and America. 



BiBL. Jones, i^oss. Estlierite, 1862, 115 j 

 Geol. May. vii. 219. 



LEANGIUM, Lk. See Dideema. 

 LEATHE'SIA, Gr^iy.— A genus of Mvrio- 



nemacone (Fucoid Algoe), consisting of glo- 

 bose or lobulated lleshyor horny structures, 

 growing upon rocks, either solid, or, by the 

 solution of the internal filamentous sub- 

 stance, ultimately hollow. Tlie fronds are 

 composed of masses of dichotomous fila- 

 ments radiating from a point ; in the olive- 

 coloured tufted species cohering laterally, 

 and forming the soft, fine coat of the lobes. 

 The sporanges are simple oval sacs attached 

 to the ends of branches of the radiating 

 filaments, between which they nestle ; or 

 multilocular, consisting of short septate fila- 

 ments occurring in similar situations, which 

 are said by Thuret to be more common ; and 

 the two kinds have not been met with 

 together. 



BiBL. Harvey, Mar. Alg. 48, pi. 10 C ; 

 Enql. Bot. pi. 1596 ; Thuret, Ann. Sc. Nat. 

 3 ser. xiv. 237, pi, 26. figs. 5-12. 



LEAVES. — The microscopic structure of 

 leaves presents a wonderful variety of con- 

 ditions, from the most simple up to very 

 complex. Instances of the former are seen 

 in the Mosses, JuNGERMANNiE^,aud other 

 Flo werless plants, where merely a simple cel- 

 lular plate exists. In the simpler leaves of 



Fig. 391. 



Vertical section pf a leaf of a Melon. 



E. S, superior epidermis ; P. S, subjacent close paren- 

 chyma; J/, infra-stomatal air-space; L, intercellular 

 space; F. v, flbro-vascular bundle (rib or vein ) ; P. i, 

 inferior lax parenchyma; S. i, inferior epidermis; P, 

 hairs ; ST, stoma. 



Magnified 100 diameters. 



Ferns, such as Hymenophylltjm, we have 

 a cellular plate traversed by vascular ribs. 

 In Sphagnum (among the Mosses) the sim- 

 ple leaves have cells containing a spiral 



