LAURENCIACEJE. 



[ 455 ] 



LEAVES. 



tetraspores are tetrahedrallr divided. The 

 antheridia are tlius described (in L. tenuis- 

 sima) by Thuret : on the smaller branches, 

 similar to those which bear the ceramidia 

 on other individuals, occur greyish 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 antherozoids 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 dichotomous hair, like those 

 common over the branches of this plant. 

 The antherozoids are elongated- ovoid, a 

 little constricted at one extremity, length 

 about 3-5000". Derbes and Seller have 

 observed them on L. pinnatifida and other 

 species. 



BIBL. Harvey, Mar. Alg. 97, pi. 12 C ; 

 Phyc. Brit. pi. 55 ; Grev. Alg. JSt\ 108, pi. 

 14 ; Derbes and Solier, Ann. So, Nat. 3 ser. 

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

 ib. ser. 4. iii. 19. 



L AURENCIA'CE^E. A family of Flo- 

 ridese. Rose-red or purple sea-weeds with 

 a cylindrical or compressed, rarely flat, li- 

 near, 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 j 2, te- 

 traspores immersed in the branches and 

 ramuli, scattered without order through the 

 surface cells ; 3, anther idia. 



British genera : 



Bonnemaisonia. Frond solid, filiform, 

 rose-red, much branched; brancheamargined 

 with subulate distichous cilia. 



Laurencia. Frond solid, cylindrical or 

 flattened, purplish or yellowish, pinnatifid, 

 ramuli blunt. 



Chrysimenia. Frond hollow, filled with 

 mucus, neither constricted nor chambered. 



Chylocladia. Branches hollow, 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. 



BEETL. Jones, Fuss. EstJierice, 1862, 115 j 

 Ge 61. Maq. vii. 219. 



LEANGIUM, Lk. See DIDEEMA. 



LEATHE'SIA, Gray. A genus of My rio- 



nernaceae (Fucoid Alge), consisting of glo- 

 bose or lobulated fleshy or horny structures, 

 growing upon rocks, either solid, or, by tha 

 solution of the internal filamentous sub- 

 stance, ultimately hollow. The 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 j and 

 the two kinds have not been met with 

 together. 



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

 Engl. Bot. pi. 1596 ; Thuret, Ann. So. 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.E,and other 

 Flo werless plants, where merely a simple cel- 

 lular plate exists. In the simpler leaves of 



Fig. 391, 



V T 



Vertical section of a leaf of a Melon. 

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

 chyma ; M, infra-stomatal air-space ; L, intercellular 

 space; F. v, nbro-vascular bundle (rib or yein ) ; P. t, 

 inferior lax parenchyma; E. t, inferior epidermis; P, 

 hairs ; ST, stoma. 



Magnified 100 diameters. 



Ferns, such as HYMENOPHYLLUM, we have 

 a cellular plate traversed by vascular ribs. 

 In SPHAGNUM (among the Mosses) the sim- 

 ple leaves have cells containing a spiral 



