70 FIRST GROUP. THALLOPHFTES. 



flat creeping lateral branches are formed with a very different mode of growth 

 from that of the sexual generation (Falkenberg, loc. ctt.} ; for the growing-point in 

 them is not formed by marginal filaments, whose hinder parts unite with one another, 

 but by a connected row of marginal initial cells. How the upright thallus of Cutleria 

 with its different mode of growth springs from the flat creeping shoots is not yet 

 ascertained ; perhaps by means of swarm-spores, which develope directly into a 

 cutleria-thallus. 



2. FUCACEAE 1 . 



The mode of fertilisation in the Fucaceae differs essentially from that of the 

 Cutleriaceae in one point only, viz. that the oosphere has here entirely lost its power 

 of motion, and is from the first a naked cell without cilia which is ejected from 

 the oogonium by the expansion of the cell-wall of that organ. But in the 

 structure 2 , growth and articulation of the thallus the Fucaceae depart widely from 

 the Cutleriaceae. As limited by Thuret they include a few genera of large marine 

 Algae, in which the vegetative body, often many feet long, is of a cartilaginous 

 texture and is attached by branched discs to stones and similar objects. It is only 

 in Fucus, Fucodium (Pelvetid), Himanthalia, and some other genera that we can 

 call the vegetative body a ' thallus ; ' in some genera, as in Sargassum, there is 

 a distinction between stem and leaf equivalent to that in the higher plants. The 

 growing-point is terminal in a depression of the apex, and the arrangement of its 

 cells varies according to the statements of observers, in the different genera. The 

 apical cell in Sargassum, Cystoszra, and others is said to be tetrahedral. In Fucus, 

 according to Rostafinski, the apex is composed of a number of cells (' initial cells ') 

 which are in shape four-sided truncated pyramids, and form divergent basal segments 

 parallel to the basal surface, and lateral segments parallel to the lateral surfaces. The 

 former are the rudiments of the cells of the medullary tissue, the latter chiefly of the rind. 



The branching of Fucus is dichotomous, and the further development is often 

 beautifully bifurcate, sometimes sympodial as in Fig. 44. The ramifications lie all 

 in one plane having only some later displacements. The surface of the tissue 

 consists of small closely-packed cells which form the rind, and continue capable of 

 division longer than cells of the medulla. The innermost layers of the rind next to 

 the elongated cells of the medullary tissue eventually produce lateral outgrowths in the 

 shape of filiform branches, which grow in between the cells of the medulla and force 

 apart those in which the walls have already become mucilaginous, so that the cells 

 ultimately lie in a compact web of these filaments (compare the medulla of Laminarieae). 

 The walls of the medullary cells are evidently formed of two different layers, an inner 

 thin firm and compact layer, and an outer mucilaginous layer very capable of 

 swelling, which fills the interstices of the cells and appears as a structureless ' inter- 

 cellular substance.' This is obviously the cause of the slimy condition of Fucaceae 

 that have lain for any length of time in fresh water. Portions of the tissue not 

 unfrequently separate from one another in the interior of the thallus, and thus form 

 cavities that are filled with air ; these project as bladders on the outer surface, and 



1 Thuret, Recherches sur la fecondation des Fucacees (Ann. d. sc. nat. IV. se'r. T. 2. 1854). 

 Pringsheim, Ueber Befrucht. u. Keimung der Algen u. d. Wesen d. Zeugungsaktes (Monatsber. d. Berl. 

 Akad. 1855). 



3 Rostafinski, Beitr. zurKenntn. der Tange, Heft I. l876.-~Reinke in Pringsheim's Jahrh. Bd. X. 



