=571.= Among the relatives of the ferns are the horsetails, so called because of the supposed resemblance of the branched stems of some of the species to a horse’s tail, as one might infer from the plant shown in fig. 325. They do not bear the least resemblance to the ferns which we have been studying. But then relationship in plants does not depend on mere resemblance of outward form, or of the prominent part of the plant. [Illustration: Fig. 321. Portion of fertile plant of Equisetum arvense showing whorls of leaves and the fruiting spike.] =572. The field equisetum. Fertile shoots.=—Fig. 321 represents the common horsetail (Equisetum arvense). It grows in moist sandy or gravelly places, and the fruiting portion of the plant (for this species is dimorphic), that is the portion which bears the spores, appears above the ground early in the spring. It is one of the first things to peep out of the recently frozen ground. This fertile shoot of the plant does not form its growth this early in the spring. Its development takes place under the ground in the autumn, so that with the advent of spring it pushes up without delay. This shoot is from 10 to 20 _cm_. high, and at quite regular intervals there are slight enlargements, the nodes of the stem. The cylindrical portions between the nodes are the internodes. If we examine the region of the internodes carefully we note that there are thin membranous scales, more or less triangular in outline, and connected at their bases into a ring around the stem. Curious as it may seem, these are the leaves of the horsetail. The stem, if we examine it farther, will be seen to possess numerous ridges which extend lengthwise and which alternate with furrows. Farther, the ridges of one node alternate with those of the internode both above and below. Likewise the leaves of one node alternate with those of the nodes both above and below. [Illustration: Fig. 322. Peltate sporophyll of equisetum (side view) showing sporangia on under side.] =573. Sporangia.=—The end of this fertile shoot we see possesses a cylindrical to conic enlargement. This is the _fertile spike_, and we note that its surface is marked off into regular areas if the spores have not yet been disseminated. If we dissect off a few of these portions of the fertile spike, and examine one of them with a low magnifying power, it will appear like the fig. 322. We see here that the angular area is a disk-shaped body, with a stalk attached to its inner surface, and with several long sacs projecting from its inner face parallel with the stalk and surrounding the same. These elongated sacs are the _sporangia_, and the disk which bears them, together with the stalk which attaches it to the stem axis, is the _sporophyll_, and thus belongs to the leaf series. These sporophylls are borne in close whorls on the axis.