[Illustration: Fig. 7. Thread of mucor, showing protoplasm and vacuoles.] Protoplasm in nitella. =17.= One of the most interesting plants for the study of one remarkable peculiarity of protoplasm is _Nitella_. This plant belongs to a small group known as stoneworts. They possess chlorophyll, and, while they are still quite simple as compared with the higher plants, they are much higher in the scale than spirogyra or mucor. =18. Form of nitella.=—A common species of nitella is _Nitella flexilis_. It grows in quiet pools of water. The plant consists of a main axis, in the form of a cylinder. At quite regular intervals are whorls of several smaller thread-like outgrowths, which, because of their position, are termed “leaves,” though they are not true leaves. These are branched in a characteristic fashion at the tip. The main axis also branches, these branches arising in the axil of a whorl, usually singly. The portions of the axis where the whorls arise are the _nodes_. Each node is made up of a number of small cells definitely arranged. The portion of the axis between two adjacent whorls is an internode. These internodes are peculiar. They consist of but a single “cell,” and are cylindrical, with closed ends. They are sometimes 5-10 cm. long. [Illustration: Fig. 8. Portion of plant nitella.] =19. Internode of nitella.=—For the study of an internode of nitella, a small one, near the end, or the ends of one of the “leaves” is best suited, since it is more transparent. A small portion of the plant should be placed on the glass slip in water with the cover glass over a tuft of the branches near the growing end. Examined with the microscope the green chlorophyll bodies, which form oval or oblong discs, are seen to be very numerous. They lie quite closely side by side and form in perfect rows along the inner surface of the wall. One peculiar feature of the arrangement of the chlorophyll bodies is that there are two lines, extending from one end of the internode to the other on opposite sides, where the chlorophyll bodies are wanting. These are known as neutral lines. They run parallel with the axis of the internode, or in a more or less spiral manner as shown in fig. 9.