226 THALLOPHYTES. 



which are set free by the wall of the mother-cell becoming gelatinous and finally de- 

 liquescing. This is evidently a more simple mode of growth than that of Vaucheria. 

 A higher degree of branching than in this latter is found in Bryopsis, which is also 

 unicellular. This genus also forms on one side root-like organs of attachment, on the 

 other upright much-branched stems (several inches in height) with unlimited apical 

 growth ; small branches with limited apical growth are formed on them in two rows 

 or spirally, which clothe the stem like leaves, and after they have detached them- 

 selves from it, fall off; while in them are formed the numerous motile zoospores^. 

 (A. Braun, /. c.) The branching of a single large cell is carried still further in the 

 genus Gaulerpa, which forms creeping stems growing at the apex with descending 

 branched rhizoids and ascending leaf-like branches^. The growth of a unicellular thallus 

 takes place in still another manner in Acetabularia. Here the plant, one or two inches 

 high, has the form of a slender Hymenomycetous Fungus, the stem of which forms a 

 rhizoid below and bears a pileus above, consisting of a disc of closely crowded rays, 

 which are themselves radial branches of the stem. This ends above in the form of a 

 boss ; at the base of the radial branches surrounding the boss stands a circle of umbel- 

 lately branched articulated hairs. In the rays of the pileus are formed the asexual spores 

 (the cell-sap contains inuline). Finally JJdotea cyathiformis must be mentioned here. 

 This species forms a stalked leaf-like thallus, the stalk | inch, the thallus from | inch 

 to 2 inches long and broad, its thickness from y^ to -^ line. As seen externally it 

 seems to consist of a cellular tissue, in reality it is composed of a regular collocation 

 of branched sacs, which, forming two cortical and one medullary layer, are all ramifica- 

 tions of one cell (Nageli, Neuere Algensysteme, p. 177)^ 



The FuCACE^ comprise, in the narrow limitation proposed by Thuret-^, a few 

 genera of large marine Algae, the thallomes of which, often many feet long, have a 

 greenish-brown colour and a cartilaginous consistency. They are fixed to stones or other 

 bodies by a branched attachment-disc. The thallomes branch dichotomously, and the 

 further development is also frequently forked, but in other cases sympodial, as in 

 Fig. 160. The ramifications, irrespectively of later displacements, all lie in one 

 plane. 



The tissue consists at the surface of small closely-crowded cells ; in the interior it is 

 laxer, and the elongated cells are often connected into articulated threads. The cell-walls 

 often consist of two clearly distinct layers, an inner thin, firm, compact layer, and an 

 outer gelatinous one, capable of swelling greatly in water, which fills up the interstices of 

 the cells, and has the appearance of a more or less structureless 'intercellular substance'; 

 it is clearly the cause of the slimy character which the Fucacese assume after lying for 

 some time in fresh water. The granular cell-contents have been but little investigated ; 

 they appear to be mostly brown, but contain chlorophyll which is concealed by other 

 colouring materials ; from dead plants cold fresh water extracts a buff-coloured sub- 

 stance •^. The tissue often becomes hollowed out internally into large cavities containing 



^ Nageli, Neuere Algensysteme. Neuenburg, 1867. 



2 Zeitschrift fur wissensch. Bot. von Nageli u. Schleiden, 1844, I. p. 134 et seq. 



^ [The remarkable fossil plant from Canada of Devonian age, Prototaxites Logani, was probably 

 an enormous Siphonaceous Alga; see W. T. Thiselton-Dyer, Journ. of Bot. 1871, p. 252, and 

 Carruthers, Monthly Micros. Journ. 1872, p. 160. — Ed.] 



* G. Thuret, Ann. des Sci. Nat. II. 1854, p. 197. 



^ In a recent paper (Comptes Rendus de rx\cad. des Sci. Feb. 22, 1869) Millardet showed that 

 from quickly-dried and pulverized Fucaceas an olive-green extract is obtained by alcohol, which, 

 shaken up with double its volume of benzine and then allowed to settle, produces an upper green 

 layer of benzine containing the chlorophyll, while the lower alcoholic layer is yellow and contains 

 phycoxanthine. Thin sections of the thallus, completely extracted with alcohol, contain also a 

 reddish-brown substance which in fresh cells adheres to the chlorophyll-grains, and can be extracted 

 by cold water, more easily when the dried Fucus has been previously pulverized. Millardet calls 



