7. MYXOPHYCEAE (Cyanophyceae or blue-green Algae), see ALGAE.
Several classes of Algae are omitted, because they include only relatively specialized forms, viz., the Diatoms (Bacillariales, q.v.), and the brown and red seaweeds (Phaeophyceae and Rhodo phyceae, see ALGAE). They are, however, likewise distinguished by their pigmentation, the products of photosynthesis, and the characteristics of the motile stages (when present), as well as by other peculiarities. No doubt each originated from simple forms parallel with those found in the classes of Protophyta, but which have become extinct or are not yet known. Should such simple forms ever be discovered they would naturally be included in the Protophyta.
Most classes exhibit a more or less exten sive range from simple unicells to branched multicellular fila ments. Examples of these varied types, as far as they occur in Isokontae, Heterokontae, and Myxophyceae, are given in the article ALGAE and there, too, will be found a consideration of their relation to one another and of the ordinary course of reproduction. One can distinguish motile and motionless unicells colonies of motile and motionless individuals, palmelloid forms with numerous cells embedded in mucilage, simple and branched filaments, etc. Analogous forms, often almost identical in shape though with the differences in pigmentation, etc., characteristic of the relevant groups are met with in most classes of holophytic Flagellata. Thus, Chrysosphaera (fig. 6) is a spherical unicell, similar to Chlorococcum (Isokontae) or Halosphaera (Hetero kontae), but it possesses the orange chromatophores and leucosin of Chrysophyceae and reproduces by zoospores closely resembling a Chromulina, one of the motile unicells of that class; similarly Hypnodinium (fig. 5) is such a motionless member of Dino phyceae, whose protoplasmic body before dividing during repro duction acquires temporarily the typical transverse and longi tudinal furrows. Palmelloid forms, analogous to Tetraspora (Isokontae), are seen in Phaeosphaera (Chrysophyceae) and Plzaeococcus (Cryptophyceae), while filamentous types are repre sented by Phaeothamnion or Thallochrysis (fig. 7) (Chryso phyceae) and Dinothrix (Dinophyceae), all reproducing by zoospores resembling closely the motile unicells of their particular classes. Further details cannot be given in the confines of a short article, but it will be clear that there is a far-going parallelism in the evolution of the different classes of Protophyta.
The reproductive processes are of the simplest kind, most commonly consisting in a mere division of the indi vidual into two parts, which in the motile forms may even take place during movement. Reproduction by zoospores, as already
mentioned, is frequent in the more advanced stationary forms (figs. 6, 7). Sexuality, altogether lacking in the Myxophyceae, is rare and restricted to the higher forms in most classes. Oogamy (see ALGAE) is encountered alone in the Isokontae, where alto gether the reproductive methods show a greater elaboration than among other Protophyta.
The definite range from simple to complex implies an upgrade evolution which is recognizable also in the reproductive processes. As far as present knowledge goes, however, the multicellular filamentous types are of the simplest kind in most Protophyta. Even when branching occurs, there is no differentiation among the cells and the formation of reproductive units takes place in the simplest possible way. In other words these classes have ended blindly without developing far in the direction of the multicellular plant and it is only in Isokontae (as well as in Phaeophyceae and Rhodophyceae which, as explained above, must have had a Protophyte ancestry) that a considerably greater specialization is found ; and this is accom panied by more complex reproductive methods and the develop ment of an oogamous sexual process. While in the two classes of seaweeds massive and complex bodies have been evolved, this is not the case in the green Algae, although in other respects their advanced forms are almost as highly specialized as those of seaweeds. The absence of more elaborate types in Isokontae is probably due to their further evolution into land-plants in the far past, just as the simpler (extinct) Phaeophyceae and Rhodo phyceae evolved into the seaweeds of the present day. In the same way other classes of Protophyta must have given rise to the various groups of fungi. There is little evidence of relationship between the known classes of Protophyta; most, if not all, seem to represent separate attempts at the evolution of a holo phytic organism. The occasional resemblances to classes of Protozoa are no doubt due to the fact that plant and animal tendencies were not clearly segregated in all the different evolu tionary series.
For a general account of Protophyta, see G. S. West and F. E. Fritsch, British Freshwater Algae (Cambridge, 1927) ; A. Fascher, Siisswasserflora Deutschlands, bsterreichs and der Schweiz (Jena, 1914 and onwards, esp. Heft 1, 2 and 4) ; and F. E. Fritsch, Presidential Address to Section K, British Association (1927).
(F. E. F.)