PROTOPHYTA. The designation Protophyta ("first plants") is applied to all simple one- and several-celled organisms that obtain their nourishment after the manner of a plant. Such forms probably afford a fairly accurate picture of what the early stages in the evolution of the vegetable kingdom were like. The simpler Algae (q.v.) are of course embraced in the Proto phyta. Together with Protozoa (q.v.) the latter constitute the Protista, which comprise all the most elementary forms of life. There arc many classes of Pro tista, some definitely holophytic (i.e., feeding like a plant), others definitely holozoic (i.e., taking in solid food like an animal), whilst still others exhibit a ming ling of plant and animal char acteristics, so that it depends to some extent on personal bias whether they be referred to Pro tozoa or Protophyta. A rigid definition is impossible and un desirable. The Protophyta may, however, justifiably be taken to include all simple organisms car rying on photosynthesis. In this process organic compounds are built up from carbon dioxide and water with the help of solar energy absorbed by pigments which are held within the cells in special protoplasmic bodies, the chromatophores. The latter always contain green chlorophyll which predominates in the green Algae (Isokontae), but in many classes is associated with other pigments (yellow, brown, red, etc.) which give a distinctive coloration to their respective members. Moreover, the carbon compounds, that accumulate in the cells after active photosyn thesis, vary in the different classes (starch, fat, leucosin, etc.), an indication that distinct metabolic processes are associ ated with the diverse types of pigmentation. The various classes can thus be distinguished on a physiological basis by their special mechanisms for nutrition.
accepted—that such sedentary forms have evolved from motile unicells in much the same way as they arise from their zoospores during their individual life-history. It is, however, probable that in some series of Protophyta (e.g., Myxophyceae) no motile organisms were ever evolved, even the unicellular individuals being motionless from the first.
In certain classes of Protophyta (Isokontae, Heterokontae, Myxophyceae) the majority of the known genera are stationary, i.e., they exhibit the essentially plant-like characteristic of immo bility. These are usually grouped under the name of Algae (q.v.) which also comprise some series that have evolved beyond the level of the Protophyta and in which no simple forms are known (cf. below). In other classes of Protophyta, however, the major ity of the species are motile and, for this reason, and also because the simpler individuals are ordinarily not clothed by a cell-wall, they have been regarded as something apart from the Algae more nearly related to Protozoa and grouped as Flagellata. But in several of these classes (Peridinieae, Chrysomonadineae) motion less unicellular and filamentous types corresponding to those found in Isokontae, Heterokontae, etc. (see ALGAE) have been discovered in recent times, and there is thus no valid reason for separating them from those Protophyta that are grouped as Algae.
A number of colourless Protista resemble plants, since they feed mainly by absorbing (organic) solutions and not by ingesting solid particles, i.e., they are saprophytes or parasites. Such forms are probably in part descended from holophytic types which have lost their chromatophores, while others may have been devoid of them from the first. From such primitively colourless Protophyta various series of fungi may have originated, although some fungi may have been derived from pigmented types by loss of chromatophores.