In the older classifications the algae proper were simply divided into four principal groups, Chlorophyceae or green algae, Cyanophyceae or blue-green algae, Phaeophyceae or brown algae and Rhodophyceae or red algae. Now, however, that more is known about the simpler organisms which used not to be regarded as algae, it has been realized that there is no real justification for such a distinction, and so the number of algal groups has been increased. This is because it has become evident that the Flagellata and other simple unicellular organisms must properly be regarded as algae, even though of a very primitive kind. At present it is most convenient to divide the algae into ten classes, one of which, the Nematophyceae, is perhaps somewhat speculative. One of the principal bases of this classification is the diff"erence in pigmentation, and a recent study of this problem shows that it is fully justified. (i) Cyanophyceae. The plants in this group show very little i I cells to plasmolysis. In some forms, principally species which are planktonic, pseudo-vacuoles may be found, and it is supposed that these contribute towards their buoyancy. The protoplast is surrounded by an inner investment which has been shown to be a 8 CYANOPHYCEAE probably do, these may be expected to differ from those of an ordinary vegetative cell. Various suggestions have been made as to their function, and in many cases they probably determine the breaking up of the trichomes (or threads) into hormogones. These hormogones are short lengths of thread which are cut off. thus forming a means of vegetative reproduction among the filamentous types. The heterocysts may also perhaps act as a food store, or they may represent archaic reproductive organs which are now functionless. It has been reported that in Nostoc and Anabaena these cells may occasionally behave as reproductive bodies. Hormogones, besides being cut off by the heterocysts, may also be produced by the development of biconcave separation disks which develop at intervals along the filament. The hormogones, together with certain of the filamentous types, exhibit a slow motion, and although ciha have been described for one species their presence has never been corroborated. The active and continual secretion of mucilage along the sides of the filaments is now regarded as the probable mechanism for securing movement. Thick-walled resting spores, or akinetes, occur in many of the filamentous forms, normally developing next to a heterocyst. The entire lack of sexuality must be ascribed to the ancient cell structure and the absence of chromosomes together, possibly, with the lack of sterols. This type of cell structure naturally provides a problem for the geneticist. There are tw^o possibilities because each cell may contain one single gene or a number of genes (organized self-reproducing bodies which determine the properties of the cell and of the organism). The genes must be separated from each other since there are no chromosomes in which they could be situated, and they will either be dis...
