The =cell= is the unit of structure of all plants. In fact the cell is the plant in many of the lower forms--so called unicellular plants. All plants, then, consist of one or more cells. While cells vary greatly in size, form, color, contents, and function, still in certain respects their structure is identical. TYPICAL CELL The typical vegetable cell is composed of a living portion or =protoplast= and an external covering, or =wall=. The protoplast includes everything within the wall. It is made up of a number of parts, each part performing certain functions yet harmonizing with the work of the cell as a whole. The protoplast (protoplasm) is a viscid substance resembling the white of an egg. The protoplast, when unstained and unmagnified, appears structureless, but when stained with dyes and magnified, it is found to be highly organized. The two most striking parts of the protoplast are the =cytoplasm= and the =nucleus=. The part of the protoplast lining the innermost part of the wall is the =ectoplast=, which is less granular and slightly denser than most of the =cytoplasm=. The cytoplasm is decidedly granular in structure. In the cytoplasm occurs one or more cavities, =vacuoles=, filled with =cell sap=. Embedded in the cytoplasm are numerous =chromatophores=, which vary in color in the different cells, from colorless to yellow, to red, and to green. The =nucleus= is the seat of the vital activity of the cell, and the seat of heredity. The whole life and activity of the cell centre, therefore, in and about the nucleus. The outer portion of the nucleus consists of a thin membrane or wall. The membrane encloses numerous granular particles--=chromatin=--which are highly susceptible to organic stains. Among the granules are thread-like particles or =linin=. Near the centre of the nucleus are one or more small rounded nucleoli. The liquid portion of the nucleus, filling the membranes and surrounding the chromatin, linin, and nucleoli, is the =nuclear sap=. Other cell contents characteristic of certain cells are crystals, starch, aleurone, oil, and alkaloids. The detailed discussion of these substances will be deferred until a later chapter.