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.
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