THALLOGENS. 101 



siphonia contains many species, some small and delicate, 

 or long and filmy, and of various tints of brown or vio- 

 let. The fronds are thread-like and jointed ; the joints 

 striped, since the stem is composed of parallel tubes or 

 siphons, from whence its name, (poly, many ; siphon, 

 tube.) The fructification is twofold, on distinct plants: 

 i.) Ceramidia, or urn-shaped cells containing pear-shaped 

 spores; 2.) Tetraspores, or groups of four spores, imbed- 

 ded in swollen branchlets. The genus Ceramium is 

 thread-like, jointed, branched, with repeated forkings. 

 The tips of the filaments are always forked, and often 

 curl toward each other. The fruit is of two kinds : 

 I.) Berries, or capsules, containing seeds, and called 

 favellce. 2.) Tetraspores, or groups of four seeds, im- 

 mersed in the substance of the branch, and surrounding 

 it in a whorl. Another beautiful and not uncommon 

 genus, found at low-water mark, or cast up after a storm, 

 is Ptilota, (from a Greek word signifying " pinnated.") 

 It has many small branches, or pinnae, and these again 

 are cut into smaller divisions, or pinnula. At the top of 

 the latter is the fructification, consisting of minute cap- 

 sules, or favellce. Some plants also contain tetraspores. 

 Corallines are a family of red Sea-weeds whose tissue is 

 consolidated by calcareous deposit. The arrangement of 

 tetraspores in the red Algae is illustrated by Fig. 36. 



6. The class of LICHENS consists of cellular plants of 

 very simple structure. They form irregular patches, 

 generally dry, upon stones, trees, etc., which they deco- 

 rate with various colors. They are found in all climates. 

 Some are used in medicine, as the Iceland Moss, (Cetra- 



ria Islandica;} others, as the Orchil, produce a valuable 

 9* 



