LATICIFEROUS TISSUE. 



[ 454 ] 



LAURENCIA. 



dese, &c. Simple unbrauclied milk-vessels 

 occur in tlae pith of the elder. 



Schacht regards them all as liber-cells. 

 They have also been considered intercel- 

 lular passages, originally devoid of a proper 

 coat, but subsequently acquiring one of vari- 

 able thickness, derived apparently from the 

 secretion which they contain ; yet the pre- 

 sence of transverse partitions would invalidate 



Fig. 388. 



Fig. 389. 



Fig. 388. Laticiferous canals from tlie root of Dan- 

 delion. Magn. 100 diams. 



Fig. 389. Laticiferous tissue extracted from Chelido- 

 niutn majus, Magn. 100 diams. 



this view. Unger, however, imagines that, 

 while some are formed in this way, they are 

 mistly developed out of confluent rows of 

 cells, like the dotted ducts ; and Trecul is 

 of opinion that they are ordinarily formed 

 in this way. Dippel considers that they 

 replace the clathrate cells of other plants. 



Canals bounded by a defined coat of cel- 

 lular tissue, forming intercellular canals or 

 ducts of very definite cliaracter, occur in the 

 ConlferK, tlie Guttiferae, Auacardiacese, &c. 

 These will be spoken of under Secreting 

 Organs of Plants. 



Canals containing a milky juice occur in 

 some of the Fungi, as in the fleshy sub- 

 stance of some Agarics, Zactarius, Sec. 



It was declared some years ago by Schultz 

 that a regular circulation of tlie latex takes 

 place through the ramified laticiferous ducts. 

 This was chiefly supported on observations 

 of movements of the latex which may be 

 made on tolerably transparent parts of 

 living plants containing these ducts. By 



bringing the uninjured sepal of Convolvulus 

 or a leaf of Chelidonhon under the micro- 

 scope (placing it in oil is advantageous in 

 the latter case), the branched latex-ducts 

 may be made out, and a flowing movement 

 of the particles may be seen occasionally. 

 But this has been shown to depend upon a 

 disturbance of the equilibriimi by external 

 causes, such as pressure and heat, and may 

 be produced at will in any direction by 

 making an incision, towards which the juice 

 flows. Trecul thinks that the laticiferous 

 canals communicate freely with the pitted 

 ducts and other vascular elements, and take 

 a share in a kind of circulation, wherein 

 they play the part of venous reser\oirs ; but 

 his views do not appear to us well founded. 



Bjbl. Schleiden, Prmcip. of Bot. 1849 ; 

 Unger, An. und Pliys. 184(3,54; Schacht, 

 Monat. Berlin Akad. 1856; Flora, 1857, 89; 

 Meyen, Secretionsorgane, 1837, 63 ; Trecul, 

 Ann. So. AW.4 ser. vii. 290; Hanstein, Milch- 

 saftgefdsse, 1864 ; Dippel, Milchs. 1865 ; 

 Vogel, Jahr. wiss. Bot. v. 31 ; Sachs, Bot. 

 88 



LAURENCIA, Lamx.— A genus of Lau- 

 renciacefe (Florideous Algse), containing 

 several British species, mostly conimou, of 

 yellowish-green, purple, or pink colour, the 

 fronds pinnately braucbed, of solid paren- 

 chymatous structure. The ceramidia are 



Fig. 390. 



Laureucia dasyphylla. 



Eamnli containing tetraspores. 



Magnifled 50 diameters. 



borne on the smaller branches, as are also 

 the antheridia ; the tetraspores are imbedded 

 in the ramuli (fig. 390). The ceramidia 

 contain tufts of pear-shaped spores; the 



