FUCUS PLATYCARPUS. 299 



Fucus platy carpus, Thuret. This species is characterised by the 

 production of male and female sexual organs in the same con- 

 ceptacle. It is further distinguished from F. vesiculosus, which it 

 otherwise closely resembles, in that it is always devoid of " air- 

 bladders," while such are very general, though not invariable, in 

 F. vesiculosus. Fertile specimens of both these species end their 

 ultimate branchlets with bladder-like swellings. These bear the 

 conceptacles. In F. platycarpus the swellings are stronger than 

 in F. vesiculosus. Cutting sections of these swollen twigs offers 

 some difficulty, on account of the strong tensions of the tissues, 

 which result in the outer edges being folded inwards. The 

 bladders collapse somewhat, while a part of the enclosed air 

 escapes audibly. The interior of the bladder appears filled with a 

 filamentous network, and partly also with colourless jelly. Cross- 

 sections prepared between elder-pith show us that the tissue of the 

 thallus has the same structure which we have already studied in 

 F. vesiculosus (p. 240) : outwardly the small polygonal cells of 

 the epidermoid layer, inwards the progressively enlarging cells of 

 the cortex, which elongate more and more, and ultimately pass 

 over into the network of threads which constitutes the medulla or 

 pith. The spaces between the threads are filled with jelly and air. 

 The conceptacles are pear-shaped hollows in the tissue. A 

 narrow opening, the osteole, communicates with the exterior, 

 and through this passes a tuft of delicate hairs. If the section 

 has cut the conceptacle in the middle line, it is easy to get an idea 

 of its structure. It is seen to be surrounded by a sheath con- 

 sisting of several layers of closely-united, tangentially elongated 

 cells. From the inner cells of the sheath arise numerous radially- 

 arranged outgrowths, extending into the conceptacle, and reducing 

 its cavity to a narrow cylindrical space, which diminishes as we 

 pass outwards. In part these structures are sterile hairs, which 

 remain unbranched. The number of these sterile hairs diminishes 

 towards the upper part of the conceptacle. The cells themselves 

 are elongated, many times as long as broad. The hairs close 

 under the osteole, on the other hand, are composed of short 

 segments. It is these hairs which protrude from the osteole in 

 a tuft. The cell-contents include protoplasm, nucleus, and very 

 small olive-green chromatophores. In structure much like these 

 barren hairs, are the much branched hairs which bear the male 

 sexual organs, the antheridia. The antheridia are unicellular 

 branches of these hairs, have an elongated elliptic form and 



