[Illustration: FIG. 219.--_Marsilia salvatrix_ (natural size): _K_ terminal bud; _b_ leaves; _f_ sporocarps; _x_ point of branching of petiole.] The two genera (with 57 species, Temperate, Tropics) are land-and marsh-plants, whose dorsiventral, creeping stem bears roots on the under surface, and the leaves in two rows on the upper side (Figs. 219, 221). The leaves of _Marsilia_ are compound, and divided into four small leaflets springing from the apex of the petiole (Fig. 219), and resemble the leaves of _Oxalis_. In the bud the leaves are circinate (Fig. 219 _b_), and at night they exhibit the well-known sleep-movements. The sporocarps are borne on the petioles of the fertile leaves, near their bases (Fig. 219 _f_); they are oblong and resemble small beans, the outer cells being hard and sclerenchymatous, while the inner ones are divided into a number of loculi arranged in two rows. On germination, water is absorbed, the two sides separate slightly, as valves (Fig. 220 _A_), and a long vermiform mass of gelatinous, parenchymatous cells (Fig. 220), swollen by the water, emerges, bearing a large number of sori arranged pinnately. Each sorus (_sr_) is covered by a thin indusium. (The thin covering may be considered an indusium physiologically, though not morphologically). [Illustration: FIG. 220.--_Marsilia salvatrix_: _A_ the sporocarp commencing to germinate; _B_ a more advanced stage of germination.] [Illustration: FIG. 221.--_Pilularia globulifera_ (natural size): _s_ sporocarps; _b_ leaves; _k_ the growing point; _r_ roots.] _Marsilia quadrifolia_, in Europe. Many species are found in Australia. The nutritious sporocarps of _M. salvatrix_ were the means of saving the Burke expedition in the interior of Australia, and hence this species has earned its specific name. _Pilularia_ has linear leaves, without lamina. The sporocarps are spheroid (Fig. 221), brown and hard, and situated near the base of the leaves. They are 2–4 chambered and open by a corresponding number of valves. Class 2. =Equisetinæ (Horsetails.)= The characteristics of this class have been described on page 204. It is divided into two sub-classes:-- 1. THE ISOSPOROUS EQUISETINÆ. To this sub-class belong, with certainty, only the EQUISETACEÆ now existent, which are represented by only one genus, _Equisetum_. 2. THE HETEROSPOROUS EQUISETINÆ. Forms which are now extinct. [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.] Sub-Class 1. =Isosporous Equisetinæ.= Order. =Equisetaceæ (Horsetails).=