Dimerous thalli ..... 
Dimerous thalli .....  |
| In dimerous thalli, there are basal and erect
filaments. There is a primary terminal initial (meristmatic cell) located at the margin of the thallus, and from which the thallus expands laterally. |
 A species of Lithophyllum from Taiwan showing cells of the basal (yellow arrows) and erect (pink arrows) filaments. |
The pseudoparenchymatous nature of the thallus is evident in dimerous
thalli. This is incorporated into the terminology used to describe the regions
of the thallus: basal and erect filaments. |
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| Under the traditional terminology that was applied to corallines, the basal filaments
were referred to as a unistratose or unilayered hypothallus, while the erect filaments were referred to as perithallus. |
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| More recently, Woelkerling (1988) makes use of the terms primigenous filaments
for what are called basal filaments here, and postigenous filaments for what are called erect filaments. The terms primigenous and postigenous relate to the origin of the filaments, rather then their orientation, but refer to the same structures. Basal and erect are much easier to relate to, particularly for non- phycologists, and I want to make coralline algae as accessible as possible to ecologists and others, so I will use basal and erect filaments here and in all my publications.
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