classes: viz., the Blue-green AlgfE {Ci/anophy-
<:ew) , Green Algte (Chlorophyccw) , Brown Algae
{Phwophyccw) , and Red Algse (lihodophycca:) .
Under the seientific names there will be found
accounts of their general habits and most strik-
ing characteristics. From the names of these
classes one might imagine that the groups are
distinguished by color marks; but in reality
fundamental morphological characters form the
basis of the classification. Although it is con-
venient to think of the AlgiE as distinguished
hy their color, there are many exceptions to the
rule, aim color should never be regarded as the
foundation of the classification.
CvAXOPiiYCE.E. Blue-green Algai are remark- able for the simplicity of their cell structure. There are probably never present distinct cell organs (nucleus, etc.), the blue-green pigment being diflfused through the outer regions of the protoplasm. It is this extreme simplicity of cell structure that seems to unite these Algae with the Bacteria. In addition to the vegetative cells, there are in the filamentous forms peculiar empty cells called Heterocysts (q.v. ), which have an important relation to the method of growth termed false branching." (See f'Y.X0PllYCE.E.) At the approach of an unfavorable season, as cold or drought, certain cells take on thick walls, becoming reproductive cells or spores. There is no method of sexual reproduction. The Cyano- ph_vce;c comprise two orders: the Coecogonales, containing the unicellular forms, and the Hor- mogouales, which comprise the filamentous types. Tile latter order takes its name from the peculiar method of vegetative reproduction, whereby the filaments break up into segments called hormo- gonia. which separate and develop new plants. CnLOROPHYCE.i;. The Green Alga:' form a large assemblage, varying from unicellular forms of very simple life history to groups with highly dift'erentiated vegetative structure and methods of sexual reproduction. It is perhaps the best group of plants for illustration of the steps and conditions of the evolution of sex. Sexual cells or Gametes (q.v.) are unquestionably derived from the asexual reproductive cells called zoo- spores or swarm spores, which are the common- est reproductive elements in the Chlorophyceae. The zoospores, after taking on sexual characters, are further differentiated into large non-motile eggs and the highly specialized sperms. One may find in the Chlorophyceae various stages in this process of sexual differentiation, and it is interesting to note that sex has arisen in several groups independently of one another. Further- more, the production by many Algie of asexual spores on the one hand, or sexual cells (gametes) on the other, is known to be determined by environmental factors, such as heat, light, and the character of the food. The same plant may be made to produce in succession non-sexual reproductive cells, or sexual elements, by merely modifying the life conditions. The most con- spicuous groups of the Chlorophyceae which may be ranked as orders are the Protococcales. Con- fervales. Conjugales, Diatomales, Siphonales, and Charales.
The Protoc8ccales include unicellular forms and the cell-colonies called Cienobia (q.v.), there being several divergent lines of develop- ment. One of the simpler types is Pleurocoecus, which grows thickly upon the north side of trees. This plant, contrary to many statements, never develops spores, and reproduces entirely by cell- division. It should not be confused with Proto- coccus (or Chlorococcus ) , a much rarer form, far more complicated in structure and life his- tory. The Protococcales are believed to repre- sent the starting point of the main line of ascent which runs through the C'onfervales to the Bryo- phytes (liverworts and mosses). There are also represented in this group several other well- marked lines, which, however, ending blindly, bear no relation to other plants. The develop- ment of these lines is determined by the degree of emphasis that is laid upon certain phases of the varied life history of the Protococcales. Tlius, the Volvocacea; have developed especially the motile condition: the TTydrodictyace:p, the colony (coenobium) condition; and the Pleuro- coccacea", the life of vegetative cells, reproducing by simple division.
The C'onfervales comprise the many-celled filamentous and membranous forms that are nearest to the theoretical main line of ascent to the Bryophytes (liverworts and mosses). The simpler types, such as Ulothrix and Ulva, have similar swimming sexual cells (gametes), which fuse (conjugate) in pairs in the water. Higher members, as CEdogonium and Coleochaete, have distiijct eggs and sperms, but there are cer- tain forms which present intermediate conditions that show clearly that the sexual differentiation of the highest types (heterogamy) has developed from the simplicity illustrated by Ulothrix, whose gametes cannot be distinguished (isoga- niy). In Coleocluete there is the further com- plication of Alternation of generations (q.v.). The fertilized egg, instead of developing directly into a new Coleocluete plant, forms a small body (sporophyte) which contains spores, each of which produces a new sexual Coleocha>te plant (gametophyte) . The appearance of this sexless generation in Coleocha>te is strikingly like the sporophyte of the simpler liverworts. The Conjugales, or Pond Scums (see Chloro- piiyce.e), are peculiar by reason of the sexually formed spore that results from the union of non- motile cells whose contents fuse directly, never pre.senting a free swimming condition. The order contains such well-known filamentous forms as Spirogyra and Zygnema, and the large group of the Desmids, the latter mostly unicellu- lar. The Conjugales are chiefly remarkable for the extreme beauty of their cell contents, the green color body being especially well developed. They are not closely related to any other group of Algae, and their origin is problematical. The method of sexual reproduction is unique, having little resemblance in manner and form to the sexual processes of other Chlorophyceie.
The Diatomales, or Diatomaceae (q.v.). have an uncertain position, but show certain affinities to the Desmids. Their color body is generally brown, but is green in some species. The Siphonales form a large group, chiefly marine. Certain members (e.g., Caulerpa) have a high grade of vegetative organization, presenting a stem-like axis that bears leaf-like expanded lateral outgrowths, and in addition colorless root-like processes (rhizoids) which grow into the substratum. The commonest illustration is Vaucheria, the only member of the order with distinct eggs and sperms (heterogamy, q.v.). The vegetative body of the Siphonales, however complex, has no partitions, but is a continuous tube or system of branching tubes through which the protoplasm slowly circulates. Such a structure
