Sentence:  Plants all marine, of an olive-brown or olive-green colour, becoming black on exposure to air; ... Frutification, tubercles contained in distinct <WSD>receptacles</WSD>, or embedded in the frond, and containing dark-coloured seeds surrounded with a pellucid limbus, which escape by a terminal pore. 
Meaning of the word marked with <WSD> tags:  A structure at the end of a branch of an alga containing conceptacles (reproductive organs).