=4. The plant spirogyra.=—This plant is found in the water of pools, ditches, ponds, or in streams of slow-running water. It is green in color, and occurs in loose mats, usually floating near the surface. The name “pond-scum” is sometimes given to this plant, along with others which are more or less closely related. It is an _alga_, and belongs to a group of plants known as _algæ_. If we lift a portion of it from the water, we see that the mat is made up of a great tangle of green silky threads. Each one of these threads is a plant, so that the number contained in one of these floating mats is very great. Let us place a bit of this thread tangle on a glass slip, and examine with the microscope and we will see certain things about the plant which are peculiar to it, and which enable us to distinguish it from other minute green water plants. We shall also wish to learn what these peculiar parts of the plant are, in order to demonstrate the protoplasm in the plant.[2] =5. Chlorophyll bands in spirogyra.=—We first observe the presence of bands; green in color, the edges of which are usually very irregularly notched. These bands course along in a spiral manner near the surface of the thread. There may be one or several of these spirals, according to the species which we happen to select for study. This green coloring matter of the band is _chlorophyll_, and this substance, which also occurs in the higher green plants, will be considered in a later chapter. At quite regular intervals in the chlorophyll band are small starch grains, grouped in a rounded mass enclosing a minute body, the _pyrenoid_, which is peculiar to many algæ. [Illustration: Fig. 1. Thread of spirogyra, showing long cells, chlorophyll band, nucleus, strands of protoplasm, and the granular wall layer of protoplasm.] =6. The spirogyra thread consists of cylindrical cells end to end.=—Another thing which attracts our attention, as we examine a thread of spirogyra under the microscope, is that the thread is made up of cylindrical segments or compartments placed end to end. We can see a distinct separating line between the ends. Each one of these segments or compartments of the thread is a _cell_, and the boundary wall is in the form of a cylinder with closed ends.