# 1911 Encyclopædia Britannica/Algae

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​ALGAE. The Latin word alga seems to have been the equivalent of the English word “seaweed” and probably stood for any or all of the species of plants which form the “wrack” of a seashore. When the word “Algae” came to be employed in botanical classification as Classifi-cation.the name of a class, an arbitrary limitation had to be set to its signification, and this was not always in keeping with its original meaning. 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. Of the fifteen genera included by Linnaeus among algae, not more than six—viz. Chara, Fucus, Ulva and Conferva, and in part Tremella and Byssus—would to-day, in any sense in which the term is employed, be regarded as algae. The excluded genera are distributed among the liverworts, lichens and fungi; but notwithstanding the great advance in knowledge since the time of Linnaeus, the difficulty of deciding what limits to assign to the group to be designated Algae still remains. It arises from the fact that algae, as generally understood, do not constitute a homogeneous group, suggesting a descent from a common stock. Among them there exist, as will be seen hereafter, many well-marked but isolated natural groups, and their inclusion in the larger group is generally felt to be a matter of convenience rather than the expression of a belief in their close inter-relationship. Efforts are therefore continually being made by successive writers to exclude certain outlying sub-groups, and to reserve the term Algae for a central group reconstituted on a more natural basis within narrower limits.
It is perhaps desirable, in an article like this, to treat of algae in the widest possible sense in which the term may be used, an indication being at the same time given of the narrower senses in which it has been proposed to employ it. Interpreted in this way, the place of algae in the vegetable kingdom may be shown by means of a table:—
| The Vegetable  Kingdom | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Cryptogamia | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$ | Thallophyta | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$ | Myxomycetes Fungi Algae |
| The Vegetable  Kingdom | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$ | Bryophyta Pteridophyta | | |
| The Vegetable  Kingdom | $\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$ | Phanerogamia | $\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$ | Gymnosperms Angiosperms | | |
Algae in this wide sense may be briefly described as the aggregate of those simpler forms of plant life usually devoid, like the rest of the Thallophyta, of differentiation into root, stem and leaf; but, unlike other Thallophyta, possessed of a colouring matter; by means of which they are enabled, in the presence of sunlight, to make use of the carbonic acid gas of the atmosphere as a source of carbon. It is true that certain Bryophyta (Marchantiaceae, Anthoceroteae) possess a thalloid structure similar to that of Thallophyta, and are at the same time possessed of the colouring matter of the Green Algae. Their life-cycle, however, the structure of the reproductive organs and their whole organization proclaim them to be Bryophyta (q.v.). On the other hand, certain undoubted animals (Stentor, Hydra, Bonellia) are provided with a green colouring matter by means of which they make use of atmospheric carbonic acid. A more important consideration is the occasional absence of this colour in species, or groups of species, with, in other respects, algal affinities. Such aberrant forms are to be regarded in the same light as Cuscuta and Orobanchaceae, for example, among Phanerogams. As these non-green plants do not cease to be classed with other Phanerogams, so must the forms in question be retained among algae. In all cases the loss of the colouring matter is associated with an incapacity to take up carbon from so simple a compound as carbonic acid.
It might be mentioned here that the whole group of the Fungi (q.v.), with its many thousands of species, is now generally regarded as having been derived from algae, and the system of classification of fungi devised by Brefeld is based upon this belief. The similarity of the morphological characters of one group of fungi to those of certain algae has earned for it the name of Phycomycetes or alga-fungi.
Further discussion of the general characters of algae will be deferred in order to take a brief survey of the subdivisions of the group. For this purpose there will be adopted the classification of algae into four sub-groups, founded on the nature the colouring matters present in the plant:—
| I. | Cyanophyceae, or Blue-green Algae. |
| II. | Chlorophyceae, or Green Algae. |
| III. | Phaeophyceae, or Brown Algae. |
| IV. | Rhodophyceae, or Red Algae. |
The merits and demerits of this system will appear during the description of the characters of the members of the several subdivisions.

## Sections

   * Cyanophyceae, or Blue-green Algae.
   * Chlorophyceae, or Green Algae.
   * Phaeophyceae, or Brown Algae.
   * Rhodophyceae, or Red Algae.
* Limits of the algae.
* Phylogeny.
* Alternation of generations.
* Polymorphism.
* Colouring matters.
* Habitat.
* Plankton.
* Benthos.
* Habit.
* Ecology.
* Uses.
* Occurrence in the rocks.
* Bibliography.