Many of the constituents given are always present in the living cells of higher plants. It cannot be too emphatically stated that where "biotic" force is manifested, these colloidal or albuminous compounds are found.
The simplest form of plant life is an undifferentiated individual, all of its functions being performed indifferently by all parts of its protoplasm.
The chemical basis of plasmodium is almost entirely composed of complex albuminous substances, and correlated with this structureless body are other compounds derived from them. Aside from the chemical substances which are always present in living matter, and are essential properties of protoplasm, we find no other compounds. In the higher organisms, where these functions are not performed indifferently, specialization of tissues is accompanied by many other kinds of bodies.
The algæ are a stage higher in the evolutionary scale than the undifferentiated noncellular plasmodium. The simple Alga protococcus[3] may be regarded as a simple cell. All higher plants are masses of cells, varying in form, function, and chemical composition.
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.
Hæatococcus and plasmodium come under the classes algæ and fungi of the Thallothyta group. The division[4] of this group into two classes is based upon the presence of chlorophyl in algæ and its absence in fungi. Gelatinous starch is found in the algæ; the fungi contain a starchy substance called glycogen, which also occurs in the liver and muscles of animals. Structureless bodies, as æthalium, 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 algæ 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 algæ 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 algæ and fungi depend upon their chemical differences.