[Illustration: FIG. 222.--_Equisetum arvense._ The prothallium
highly magnified. _A_ Male; _s, s_ antheridia. _B_ Portion of a
female, cut through vertically; _œ œ_ archegonia, the central one
is fertilised; _h h_ root-hairs.]
[Illustration: FIG. 223.--_Equisetum maximum._ Spermatozoids: _a_
shows them still enveloped by the mother-cell.]