While exceptions are known, members of the Florideophyceae generally share a majority of the following characteristics:

• a multicellular thallus with apical growth
• the presence of pit connections (Figure 1)
• a filamentous gonimoblast
• tetrasporangia
• cells with multiple nuclei and plastids (Figure 2)
• life histories that are some variation of a triphasic alternation of generations (Figure 3)

Figure 1. Electron micrograph of a Cumagloia andersonii (Nemaliales) pit plug. The pit plug occupies an aperture in the cell wall (W) and consists of a plug core (Co), flanked on either end by a thin inner cap layer (IC), and a plate-like outer cap layer (OC).
Image copyright © 2000, C.M. Pueschel.

Figure 2. DAPI stained cell of Agardhiella subulata showing multiple brightly fluorescing nuclei.
Image copyright © 2000, D.F. Kapraun.

Figure 3. Diagram of triphasic life history. Haploid (1N) male gametophytes produce spermatia that are released, while haploid female gametophytes produce carpogonia (=egg cells) that are retained on the female gametophyte. After fertilization, the diploid (2N) zygote is still retained on the female gametophyte and develops into the diploid carposporophyte. The carposporophyte produces diploid carpospores that are released and develop into the diploid tetrasporophyte. The tetrasporophyte produces tetrasporangia where meiotic divisions result in haploid tetraspores. These tetraspores then develop into haploid gametophytes completing the life cycle.
Image copyright © 2000, D. W. Freshwater.