A typical living cell may be described as composed of a cell wall and contents.  The cell wall is a firm, elastic membrane closed on all sides, and consists mainly of cellulose, water, and inorganic constituents.  The contents consist of a semi-fluid colloidal substance, lying in contact with the inner surface of the membrane, and, like it, closed on all sides.  This always is composed of albuminous substances.  In the higher plants, at least, a nucleus occurs embedded in it; a watery liquid holding salts and saccharine substances in solution fills the space called the vacuole, inclosed by the protoplasm.

These simple plants may be seen as actively moving cells or as non-motile cells.  The former consist of a minute mass of protoplasm, granular and mostly colored green, but clear and colorless at the more pointed end, and where it is prolonged into two delicate filaments called cilia.  After moving actively for a time they come to rest, acquire a spherical form, and invest themselves with a firm membrane of cellulose.  This firm, outer membrane of the Protococcus accompanies a higher differentiation of tissue and localization of function than is found in the plasmodium.

Haeatococcus and plasmodium come under the classes algae and fungi of the Thallothyta group.  The division[4] of this group into two classes is based upon the presence of chlorophyl in algae and its absence in fungi.  Gelatinous starch is found in the algae; the fungi contain a starchy substance called glycogen, which also occurs in the liver and muscles of animals.  Structureless bodies, as aethalium, contain no true sugar.  Stratified starch[5] first appears in the phanerogams.  Alkaloids have been found in fungi, and owe their presence doubtless to the richness of these plants in nitrogenous bodies.

In addition to the green coloring matter in algae are found other coloring matters.[6] The nature[7] of these coloring matters is usually the same through whole families, which also resemble each other in their modes of reproduction.

In form, the algae differ greatly from filaments or masses of cells; they live in the water and cover damp surfaces of rocks and wood.  In these they are remarkable for their ramifications and colors and grow to a gigantic size.

The physiological functions of algae and fungi depend upon their chemical differences.

These facts have been offered, simple as they are, as striking examples of chemical and structural opposition.

The fungi include very simple organisms, as well as others of tolerably high development, of most varied form, from the simple bacillus and yeast to the truffle, lichens, and mushrooms.

The cell membrane of this class contains no pure cellulose, but a modification called fungus cellulose.  The membrane also contains an amyloid substance, amylomycin.[8] Many of the chemical constituents found in the entire class are given in Die Pflanzenstoffe.[9]