None of the existing Red Seaweeds (Rhodophyceae) has a unicellular body.
Many of the lower forms of Brown Seaweeds (Phoeophyceae) have a thallus consisting of simple or branched cell threads, as in the green and red forms. The lateral union of the branches to form a solid thallus is not, however, so common, nor is it carried to so high a pitch of elaboration as in the Rhodophyceae.
Some bulky Brown Seaweeds, where assimilation is strongly localized, some of the deep cells are highly specialized for the latter function.
The group, the exceptions being met with almost entirely among the higher Brown Seaweeds, in which is found parenchyma produced by the segmentation of an apical cell of the whole shoot, or by cell division in some other type of meristem.
Filamentous diatoms may be mounted like ordinary seaweeds, and, as well as all parasitic algae, should whenever possible be allowed to remain attached to a portion of the alga on which they grow, some species being almost always found found parasitical on particular plants.
Rhamnose or isodulcite, a component of certain glucosides, fucose, found combined in seaweeds and chinovose, present as its ethyl ester, chinovite, in varieties of quina-bark, are methyl pentoses.
It is also found in small quantities in sea-water, in some seaweeds, and in various mineral and medicinal springs.
Over seventy varieties of seaweeds, growing in the fresh-water pools and in the waters near the coast, are used by the natives as food.
The absence of differentiation into root, stem and leaf which prevails among seaweeds, seems, for example, to have led Linnaeus to employ the term in the Genera Plantarum for a sub-class of Cryptogamia, the members of which presented this character in a greater or less degree.
As in the case of epiphytic brown seaweeds, the rhizoids of the epiphyte often penetrate the substance of the supporting alga.
The ash of seaweeds, known in Scotland as kelp, and in Brittany as varec, was formerly used as a source of iodine to a greater extent than is at present the case.
Murray, Phycological Memoirs (London, 1892-1895); An Introduction to the Study of Seaweeds (London, 1895); C. Naegeli, Die neueren Algensysteme (Zurich, 1847); F.