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 algae, are characterized by the deposition of 

 large amounts of carbonate of lime, which makes them 



FIG. 19 (Red Algje). A, Callithamnion floccosum, a simple red sea-weed, 

 showing the protoplasmic 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. 



