28:j THALLOPHVTES. 



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 

 air which are forced outwards and serve as swimming bladders. The thallus has not, 

 as far as I know, been further minutely examined ^ ; the outer conformation especially 

 has been but little investigated from a morphological point of view. (Gf. Nageli, Neuere 

 Algensysteme.) 



The mode of sexual reproduction is far better known through the labours of Thuret 

 and Pringsheim. The antheridia and oogonia are formed in spherical hollows (Con- 

 ceptacles) which make their appearance in large numbers and densely crowded at the 

 ends of the longer forked branches or of lateral shoots of peculiar form. These con- 

 ceptacles are not formed in the interior of the tissue, but are depressions of the sur- 

 face which become walled in by the surrounding tissue and so overgrown that at 

 length only a narrow channel remains, opening outwards. The layer of cells which 

 clothes the hollow is thus a continuation of the external epidermal layer of the thallus ; 

 and since the filaments which produce the antheridia and oogonia sprout from it, these 

 latter are, morphologically, trichomes. Some species are monoecious, i.e. both kinds 

 of sexual organs are developed in the same conceptacle, as in Fucus platycarpus (Fig. 

 184) ; others are dioecious, the conceptacles of one plant containing only oogonia, those 

 of another plant only antheridia {e.g. Fucus 'vesicu/osus, serratus, and nodosus, Himanthalia 

 lorea). A number of hairs which grow in the conceptacles among the sexual organs are 

 long, slender, articulated, but unbranched, and project in F. platycarpus out of the mouth 

 of the receptacle in the form of tufts (Fig. 184, B). The antheridia are produced as 

 lateral ramifications of branched hairs. Each antheridium consists of a thin-walled oval 

 cell, the protoplasm of which splits up into numerous small antherozoids ; these are 

 pointed at one end, and each is furnished with two motile cilia ; in the interior they 

 contain a red spot. The formation of the oogonium begins with the papillose swelling of 

 a parietal cell of the conceptacle ; the papilla is separated off by a septum, and divides, 

 as it grows in length, into two cells, a lower, the pedicel-cell, and an upper, which 

 forms the oogonium ; this swells up into a spherical or ellipsoidal form and becomes 

 filled with dark protoplasm. The protoplasm of the oogonium remains undivided in 

 some genera {Pycnophycus, Himanthalia, Cystoseira, Halidrys), and the whole contents 

 of the oogonium thus form one oosphere ; in others [Pel'vetia) it divides it into two, four 

 {0%othallia 'vulgaris), or eight {Fucus). Fertilisation takes place outside the concep- 



* In a recent paper (Comptes Rendus de I'Acad. des Sci. Feb. 22, 1869) Millardet showed that 

 from quickly-dried and pulverised Fucaceae 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 pulverised. Millardet calls 

 this reddish-brown substance phycophoeine. (Compare further the interesting treatise of Rosanoff, 

 Observations sur les fonctions et les proprietes des pigments de diverses Algues, in Memoires de la 

 Societe des Sci. Nat. de Cherbourg, vol. XIII. 1867 ; and Askenasy, Bot. Zeitg. no. 47, 1869.) [^^^ 

 also Sorby, Proc. Roy. Soc. 1873, vol. XXI, pp. 445, 454, 461.] 



2 [See Rostafinski, Beit. z. Kenntniss der Tange, 1876: also Bower, Quart. Journ. Mic. Sci. 

 vol, 20, new series.] 



