which these lines consist, will be found to move slowly and steadily, the motion becoming perceptible by means of the minute granules of which the protoplasm consists. The currents return at the ends of the cell, there being no passage of the contents of one cell into the cavity of either of those adjacent. During this examination, it will be noticed that the surface of the cell-wall is striated with fine wrinkles. It may be remarked that the hairs should be taken from flowers which have only just opened; for this curious and inexplicable rotation is connected with the growth of the cell; and when this has attained maturity, it no longer occurs. The phenomenon may be observed in many other hairs of plants, as those of common groundsel (_Senécio vulgáris_) (Pl. I. fig. 12 _a_, _b_), and in the cells of the leaves of some water-plants; but I must refer to the article “Rotation” in the Dictionary for further information. The most important variety of hair is that derived from the Cotton-plant (a kind of Mallow), and forming the cotton of commerce. These hairs spring from the epidermis of the seeds. The cells composing it are very long and soft, becoming flaccid and easily bent when dry (Pl. IX. fig. 13). _Stings._--Stinging hairs or stings may be well illustrated by reference to the common large nettle (_Urtíca dioíca_). In this plant they consist of a thick-walled cell, bulbous at the base, which is imbedded in the epidermis (Pl. I. fig. 21), the pointed end being terminated by a very minute dilatation or knob. The sting contains an acrid liquid, which escapes when the little knob is broken off in wounding the skin, and produces the well-known irritation. By the side of the figure of the sting is represented the point of a fine needle (fig. 20), showing that the expression “sharp as a needle” has no force when microscopic bodies are in question. _Stom´ata_ (στὁμα, mouth).--On viewing a strip of epidermis, the observer will be sure to notice certain oval or roundish bodies (Pl. I. fig. 13 _a_), composed of mostly two kidney-shaped cells in apposition but leaving a chink between them; these are the stomata. They communicate beneath with the intercellular passages, of which they may be considered the mouths; and by their agency a direct communication is established between these passages and the air. The two cells which guard the orifice are termed the “guard cells.” Stomata are most numerous on the under surface of leaves; they are entirely absent in plants growing under water, and in most of the lower plants. In many of the stomata, viewed in the ordinary way, the air situated between the guard cells is indicated by the black spot or dot present; but after a time, or by the application of a gentle heat to the slide, the air becomes displaced by the water, and their structure becomes very distinct.