LAURENCIA. 



[ 380 ] 



LEAVES. 



LAURENCIA, Lamx. — A genus of Lau- 

 renciacese (Florideous Algae), containing 

 several British species, mostly common, of 

 yellowish-green, purple or pink colour, the 

 fronds pinnately branched, of solid paren- 

 chymatous structure. The ceramidia are 

 borne on the smaller branches, as are also the 

 antheridia ; the tetraspores are imbedded in 

 the ramuli (fig. 394). The ceramidia contain 



Fig. 394. 



Laurencia dasyphylla. 



Ramuli containing tetraspores. 



Magnified 50 diameters. 



tufts of pear-shaped spores ; the tetraspores 

 are tetrahedrally divided. The antheridia 

 are thus described (in h. tenuissima) by 

 Thuret : on the smaller branches, similar 

 to those which bear the ceramidia on other 

 individuals, occur greyish convoluted plates 

 of cellular tissue, of irregular form, bordered 

 by a line of roundish cells, containing gene- 

 rally a yellow liquid. Hyaline cells contain- 

 ing antherozoids are implanted 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". MM, Derbes and 

 Solier have observed them on L.pinnatifida 

 and other species. 



BiBL. Harvey, Brit. Mar. Alg. p. 97. 

 pi. 12 C ; Phyc. Brit. pi. 55, &c. ; Grev. Alg. 

 Brit. p. 108. pi. 14 ; Derbes and Solier, 

 Ann. d. Sc. nat. 3 ser. xiv. p. 276, pi. 37 ; 

 Thuret, id. xvi. p. 65. pi. 7? id. ser. 4. iii. 

 p. 19. 



LAURENCIACEiE.— A family of Flo- 



rideae. Rose-red or purple sea-weeds with 

 a cylindrical or compressed, rarely flat, Hnear, 

 narrow, areolated, inarticulate or constricted 

 and chambered, branching frond, composed 

 of polygonal cells. Fructification ; 1, con- 

 ceptacles {ceramidia) external ovate, fur- 

 nished with a terminal pore, and containing 

 a tuft of pear-shaped spores ; 2, tetraspores 

 immersed in the branches and ramuli, 

 scattered without order through the surface 

 cells ; 3, antheridia. 



Synopsis of the British Genera. 



I. BoNNEMAisoNiA. Frond solid, fili- 

 form (rose-red), much branched; the branches 

 margined with subulate, distichous cilia. 



II. Laurencia. Frond solid, cylindri- 

 cal or compressed (purplish or yellowish), 

 pinnatifid ; the ramuli blunt. 



III. Chrysymenia. Frond hollow, 

 filled with w^atery mucus, neither constricted 

 nor chambered. 



IV. Chylocladia. Branches hollow, 

 filled with watery mucus, constricted at 

 intervals and chambered. 



LEANGIUM, Lk. See Diderma. 



LEATHESIA, Gray.~A genus of Chor- 

 dariacei (Fucoid Algae), consisting of globose 

 or lobulated fleshy or horny structures, 

 growing upon rocks, either solid, or, by the 

 solution of the internal filamentous sub- 

 stance, ultimately hollow. The fronds are 

 composed of masses of dichotomous filaments 

 radiating from a point, in the olive- coloured 

 tufted species cohering laterally, and forming 

 the soft, fine coat of the lobes. The oospo- 

 ranges are oval sacs attached at the ends of 

 branches of the radiating filaments, between 

 which they nestle ; the trichosporanges, con- 

 sisting of short septate filaments occurring 

 in similar situations, are said by Thuret to 

 be more common, and the two kinds have 

 not been met with together. 



BiBL. Harvey, Br. Mar. Alg. p. 48. 

 pi. 10 C ; Engl. Bot. pi. 1596 ; Thuret, Ann. 

 des Sc. nat. 3 ser. xiv. p. 237. pk 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^ and 

 other Flowerless plants where merely a 

 sim pie cellular plate exists. In the simpler 

 leaves of Ferns, such as Hymenophyl- 

 LUM, we have a cellular plate traversed by 

 vascular ribs. In Sphagnum among the 

 Mosses, the simple leaves have cells contain- 



