This section is from the book "Plants And Their Uses - An Introduction To Botany", by Frederick Leroy Sargent. Also available from Amazon: Plants And Their Uses; An Introduction To Botany.
Part 177. The seaweed subdivision, algae in general. It is believed by evolutionists that life originated in the sea. Among the algae we generally find that the marine forms are more primitive than their nearest relatives growing in fresh water or in the air. Hence, as being at once the most primitive and most typical of the algae, seaweeds may not inappropriately serve to name the entire group. Over 12,000 species of algae are known. In spite of the great variety of form in the plant-body and in the life-histories of various algae, 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 algae 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 algae 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.
 
Continue to: