ALG IN GENERAL 491 
described in our examples of brown alge, and many of the 
red seaweeds rival the brown in elaborate forms of thallus 
simulating remarkably the shoots of higher plants. 
177. The seaweed subdivision, alge in general. It is 
believed by evolutionists that life originated in the sea. 
Among the alge we generally find that the marine forms are 
more primitive than their nearest relatives growing in fresh 
Fic. 321.—Carrageen (see also Fig. 118). A, transverse section through a 
fruiting branch showing the spore-clusters embedded in the thallus, £2. 
B, same, 272, showing rind (r), pith-like interior (m), and spore-clusters 
(s). (Luerssen.) 
water or in the air. Hence, as being at once the most primi- 
tive and most typical of the alge, seaweeds may not inappro- 
priately serve to name the entire group. Over 12,000 species 
of alge are known. In spite of the great variety of form in 
the plant-body and in the life-histories of various alge, an 
alga may generally be recognized as a plant without true roots, 
stems, or leaves, but containing chlorophyll, although the leaf- 
green color may be masked by some other pigment. 
It must not be supposed that the pigments which have suggested 
names for the several classes of algee are invariably present in these 
groups, or that mere color is here the basis of classification. The 
pigments in question happen to be associated very generally with 
fundamental peculiarities of structure and life-history which give 
evidence of kinship; hence alge of the same color may as a rule be 
regarded as akin and thus the pigments afford a convenient though 
superficial mark for recognizing related forms. 
