2. The heterosporous Equisetinæ. Forms which are now extinct.
Sub-Class 1. Isosporous Equisetinæ.
Order. Equisetaceæ (Horsetails).
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.
Fig. 223.—Equisetum maximum. Spermatozoids: a shows them still enveloped by the mother-cell.
The sexual generation. The prothallium is green and leaf-like, as in the majority of Ferns, but irregularly branched and curled. It is often unisexual. The male prothallia bear antheridia only, and are smaller and less branched (Fig. [222] A) than the female; the latter may attain a diameter of ½ an inch, and bear archegonia only (Fig. [222] B). The antheridia and the archegonia resemble those of the Ferns, but the spermatozoids (Fig. [223]) are larger and less twisted. On the last curve is situated a more or less elongated appendage of cytoplasm (Fig. [223] c).
The asexual generation. The embryo is similar to that of the Ferns. The fully developed Equisetum is a perennial herb, with widely creeping (in some species tuberous) rhizome, from which extend erect, aerial, most frequently annual shoots.
