| Melobesieae. | Thallus encrusting, leaf- or coral-like; unsegmented. (Melobesia, Lithophyllum, Lithothamnion.) |
| Corallineae. | Cylindrical filamentous and segmented thallus. (Amphiroa and Corallina.) |
The genus Corallina is the best known British representative of the Corallinaceae. With other members of the group it was long regarded as a coralline animal, and it is only comparatively recently that the plant-nature of these forms has been generally admitted. Lithophyllum, Lithothamnion, Melobesia, and other genera of the Corallinaceae and some of the Siphoneae play a very important part in the building and cementing of coral-reefs. The pink or rose-coloured calcareous thallus of some of these calcareous algae or Nullipores imparts to coral-reefs a characteristic appearance. In some cases, indeed, the coral-reefs are very largely composed of algae. Saville Kent[345] describes the Corallines or Nullipores of the Australian Barrier-reef as furnishing a considerable quota towards the composition of the coral rock. Mr Stanley Gardiner, who accompanied the coral-boring expedition to the island of Funafuti, has kindly allowed me to quote the following extract from his notes, which affords an interesting example of the importance of calcareous algae as reef-building organisms. “It is quite a misnomer to speak of the outer edge of a reef like this (Rotuma Island) as being formed of coral. It would be far better to call it a Nullipore reef, as it is completely encrusted by these algae, while outside in the perfectly clear water, 10 to 15 fathoms in depth, the bottom has a most brilliant appearance from masses of red, white and pink Nullipores, with only a stray coral here and there.”
Agassiz[346] has given an account of the occurrence of immense masses of Nullipores (Udotea, Halimeda etc.) in the Florida reefs; his description is illustrated by good figures of these algae.
In the Mediterranean there are true Nullipore reefs, which are interesting geologically as well as botanically. Walther[347] has described one of these limestone-banks in the Gulf of Naples which occurs about 1 kilometre from the coast and 30 metres below the surface of the water. Every dredging, he says, brings up numberless masses of Lithothamnion fasciculatum (Lamarck), and L. crassum (Phil.). Between the branches of the algae, gasteropods and other animals become completely enclosed by the growing plants, while diatoms, foraminifera, and other forms of life are abundant. Water percolating through the mass gradually destroys the structure of the algal thalli, and in places reduces the whole bank to a compact structureless limestone.
The same author[348] has also called attention to the importance of Lithophyllum as a constructive element in the coral-reefs off the Sinai peninsula.
Lithothamnion a typical genus of the Corallinaceae may be briefly described.
Lithothamnion. Fig. 37.
Philippi[349] was the first writer to describe this and other genera as plants. He gave the following definition of Lithothamnion:
“Stirps calcarea rigida, e ramis cylindricis vel compressiusculis dichotoma ramosis constans.”
The thallus of Lithothamnion grows attached to the face of a rock or other foundation, and forms a hard, stony mass, assuming various coralline shapes. The exposed face may have the form of numerous short branches or of an irregular warty surface.