==The Cell==.—­But what is this cell which forms the unit of life, and to which all the fundamental vital properties can be traced?  We will first glance at the structure of the cell as it was understood by the earlier microscopists.  A typical cell is shown in Fig. 9.  It will be seen that it consists of three quite distinct parts.  There is first the cell wall (cw) which is a limiting membrane of varying thickness and shape.  This is in reality lifeless material, and is secreted by the rest of the cell.  Being thus produced by the other active parts of the cell, we will speak of it as formed material in distinction from the rest, which is active material.  Inside this vesicle is contained a somewhat transparent semifluid material which has received various names, but which for the present we will call cell substance (Fig. 9, pr).  It may be abundant or scanty, and has a widely varying consistency from a very liquid mass to a decidedly thick jellylike substance.  Lying within the cell substance is a small body, usually more or less spherical in shape, which is called the nucleus (Fig. 9, n).  It appears to the microscope similar to the cell substance in character, and has frequently been described as a bit of the cell substance more dense than the remainder.  Lying within the nucleus there are usually to be seen one or more smaller rounded bodies which have been called nucleoli.  From the very earliest period that cells have been studied, these three parts, cell wall, cell substance, and nucleus have been recognized, but as to their relations to each other and to the general activities of the cell there has been the widest variety of opinion.

[Illustration:  FIG. 9.—­A cell; cw is the cell wall; pr, the cell substance; n, the nucleus.]

==Cellular Structure of Organisms==.—­It will be well to notice next just what is meant by saying that all living bodies are composed of cells.  This can best be understood by referring to the accompanying figures.  Figs. 10-14, for instance, show the microscopic appearance of several plant tissues.

[Illustration:  FIG. 10.—­Cells at a root tip.]

[Illustration:  FIG. 11.—­Section of a leaf showing cells of different shapes.]

At Fig. 10 will be seen the tip of a root, plainly made of cells quite similar to the typical cell described.  At Fig. 11 will be seen a bit of a leaf showing the same general structure.  At Fig. 12 is a bit of plant tissue of which the cell walls are very thick, so that a very dense structure is formed.  At Fig. 13 is a bit of a potato showing its cells filled with small granules of starch which the cells have produced by their activities and deposited within their own bodies.  At Fig. 14 are several wood cells showing cell walls of different shape which, having become dead, have lost their contents and simply remain as dead cell walls.  Each was in its earlier history filled with cell substance and contained a nucleus.  In a similar way any bit of vegetable tissue would readily show itself to be made of similar cells.