72 EVOLUTION OF PLANTS 
to the action of the waves, are comparatively large, and 
tough and leathery in consistence like the kelps, which 
they also resemble in the general arrangement of the 
cells in the thallus. One peculiar family, the so-called 
Coralline algw, are characterized by the deposition of 
large amounts of carbonate of lime, which makes them 
a, 
Fia. 19 (Red Alge). — A, Callithamnion floccosum, a simple red sea-weed, 
showing the Lipok age oe connections between the cells, and the non- 
sexual reproductive bodies, tetraspores, sp; B, a single tetraspo- 
rangium with the four contained spores; C, the spore-fruit, or cystocarp 
of a somewhat more complicated form, Polysiphonia; D, one of the 
larger red sea-weeds, Gigartina spinosa, reduced about one-half. 
resemble corals in form, and in their stony hardness. 
Some of these often grow associated with true corals, 
and play an important part in the building up of coral 
reefs. Like the true corals, these corallines have been 
preserved very perfectly in a fossil condition, and they 
appear to be quite ancient forms. 
