Melosira dickiei

Orthoseira Dickieii, n. sp. Pl. XII. fig. E 1-7 (220/1). Filamentis brevibus ; cellulis laevissimis. The filaments of this beautiful species consist generally each of from two to four frustules, which are hyaline and perfectly smooth, and each with its central cavity filled with a dark red-brown endochrome. The sporangium of Orthoseira Dickieii is no less beautiful than interesting : it is fusiform in shape and marked with numerous annular constrictions, each with a corresponding internal septum or chamber, the origin of which can only be understood by paying attention to the early development of the sporangium. In fig. E3 is shown a filament of this species, the terminal cells of which have each commenced to develop a sporangium ; E4 represents two such cells or young sporangia ; and E5 a mature sporangium. It will be observed that the formation of the ring-like markings is progressive, and that they go on increasing in number until the sporangium is fully developed. At the commencement of the formation of the sporangium, the endochrome, at the same time that it withdraws from the end of the frustule, produces at its centre an additional ring of cell-membrane ; and this process continuing to take place at certain intervals—each new ring of cell-membrane exceeding in diameter those previously formed—produces at length the structure represented in E5. Or it may be a more correct explanation of the process to say, that an entire new cell-membrane has been developed by the young sporangium at the time each new ring has been formed, and that thus have originated the several chambers into which the ends of the sporangium are divided. Fissiparous division of the sporangium subsequently takes place, as shown in fig. E6, and sporangial frustules are developed from each half, E7.

• Basionym
  Orthoseira dickiei
• Author
  Thwaites 1848