Thallus organization in coralline algae

Example of vertical section through the thallus of a typical nongeniculate coralline alga. Note that the thallus is constructed from the aggregation filaments, and is therefore pseudoparenchymatous.

Being algae, the plant body in corallines is referred to as a thallus (pl. thalli). Most coralline algae have an internal anatomy of the thallus that is based on the aggregation of branching filaments, each of which is formed through the action of its own single meristematic cell. This type of thallus anatomy is said to be pseudoparenchymatous. Only certain parasitic species, and some epiphytes, produce filaments that may not be aggregated into pseudoparenchyma. Since their anatomy is based on the elaboration of filaments, true tissues do not occur in coralline algae, and it is therefore incorrect to use the term ‘tissue’ do describe regions within the thallus.

Most nongeniculate corallines show some degree of dorsiventral arrangement, that is, an arrangement in which a distinct upper and lower surface can be distinguished. There are two different types of organization in dorsiventral thalli:
dimerous organization;
monomerous organization.

In dimerous thalli, there are two distinct groups of filaments oriented more-or-less at right angles to one another, with a ventral layer of filaments (called basal filaments) that is uually one cell thick, and an erect layer in which filaments (called erect filaments) vary from one to many cells in length and comprise most of the thallus thickness. Dimerous thalli arise when a spore germinates to produce a unistratose layer of radiating, repeatedly branched filaments, that grow via meristematic cells that are located at the apices of the branches. The growth of this layer of basal filaments contributes to the areal expansion of the thallus. Erect filaments are formed in most dimerous species when cells of the basal filaments divide periclinally to form erect filaments. The first cell that is cut off remains meristematic and forms new cells of the erect filament ventrally, and epithallial cells dorsally. Since it is usually overlain by one or more epithallial cells, the meristematic cell is usually referred to as a subepithallial initial. Dimerous thalli occur in Lithophyllum, Titanoderma, Exilicrusta, Melobesia, and some species of Hydrolithon and Pneophyllum.

In monomerous thalli, there is but a single pseudoparenchymatous system of repeatedly branched filaments in which some derivatives contribute to a medulla which runs more-or-less parallel to the thallus surface (or substratum in dorsiventral, attached species) and some derivatives curve outwards and collectively form a cortex. In most dorsiventral thalli, downwards-curving filaments are suppressed, and only occur vestigially. The cortical filaments terminate at the upper surface in one or more epithallial cells, that are subtended by a subepithallial initial. Continued division of the subepithallial initial increases the thallus girth, and may give rise to surface protuberances which may themselves branch. Monomerous thalli occur in Clathromorphum, Leptophytum, Lithothamnion, Phymatolithon, Mesophyllum, and many species of Hydrolithon, Spongites, and Pneophyllum. In some species, the primary thallus is dimerous, but monomerous regions may develop secondarily from the cells of erect filaments (e.g. Lithophyllum incrustans).

In both monomerous and dimerous forms, the cells of adjacent filaments below the merismetic cells often fuse together, or form pit connections, allowing for cellular interchange throughout the plant in spite of the calcified cell walls. Cell fusions and secondary pit connections are used for taxonomic purposes at the sub-family as well as species level.