"Oh! papa," said Jack, "what is this curious plant that grows so abundantly on the grass here? I know it well by sight, but do not know its name." It is a spike of horse-tail; see how the stem is marked with lines, and how curiously jointed it is, and quite hollow except where the joints occur. The fruit is borne at the top of the plant (_a_); see, as I shake it, what a quantity of dust comes from it; this dust is the fruit, or spores as they are called; each spore is of an oval form, with four elastic threads. If I were to put some of this dust on a glass slide, and look at it under the microscope, I should see a curious sight. The four threads would be spread out, but if I were to breathe on the glass, these threads would coil themselves round the oval body; but as soon as the effect of the moisture had passed away, the threads would shoot out again in the same position as they were at first, causing the spore to leap as if it were alive. The stems are of two kinds, fertile and unfertile; the one you have in your hands is a fertile spike, and appears only in the spring; the unfertile ones have no dust-like fruit, and have numerous jointed branches growing in rows, or whorls as they are termed, round them; they remain throughout the summer, and in some places form quite a thick cover. Feel how rough the stem is; this is due to the presence of a quantity of silex or flint in it; on this account some of the species are used for polishing purposes. One kind, under the name of "Dutch rushes," is imported from Holland, being used for polishing mahogany, ivory, metal, &c. The horse-tails for the most part grow in moist ground, in ditches and on the borders of lakes; some, however, are common in corn fields and on the roadside. In this country they do not attain a height of more than a few feet, but in tropical countries one or two species grow to the height of sixteen feet or more. [Illustration: HYDRAE, ON ROOTS OF DUCKWEED.]