Zostera (Grass-wrack) is an entirely submerged, marine plant with creeping rhizome (with displacement of buds) and strap-shaped leaves. The flowering shoots are sympodia with displacement of the axes (Fig. [281]). The inflorescence is a peculiar, flatly-compressed spike, on one side of which the flowers are borne (Fig. [281]). This inflorescence may be considered, no doubt correctly, to be derived from the symmetrical spike of Potamogeton by strongly dorsiventral development, and by a strong suppression of the floral parts taking place simultaneously. Two rows of flowers are developed, but of these one is so pressed into the other that apparently only one is present. Each flower consists of only 1 stamen and 1 carpel situated at the same height (Fig. [281]); the unilocular ovary encloses 1 pendulous ovule and bears a bifid style. As regards the perianth (?) one leaf may be present (Z. nana, Fig. [281] D). The pollen-grains are filamentous. Pollination takes place under water. Posidonia and Cymodocea are allied to these. About 70 species.
Fig. 281.—Zostera. A Diagram of the branching of the floral shoots: I, II ... are the successive shoot-generations, every other one being shaded; g1 g2 ... fore-leaves; sp1 sp2 ... spathes for the successive spikes. Each shoot is united for some distance with the parent axis (indicated by the half-shaded internodes). Each shoot commences with a fore-leaf turning towards the parent axis, g; succeeding this is the spathe, sp; and then the inflorescence. The fore-leaf supports a new lateral shoot. B Diagram of a shoot, II, which is borne laterally in the axil of the fore-leaf g1, on the shoot I, g2 its fore-leaf; sp2 its spathe; sti squamulæ intravaginales. II Is the spadix with stamens and carpels; b a perianth-leaf (or connective expansion, similar to those which occur in Potamogeton). C The upper portion of a young spadix with development of flowers. D Part of a spadix with 2 flowers; the parts which theoretically belong to one another are connected by a dotted line.
Order 3. Aponogetonaceæ. Aquatic plants with tuberous stem. They have a single, petaloid perianth (3–2–1–leaved), most frequently 6 stamens and 3(-6) carpels. Straight embryo.—About 15 species (Africa, Madagascar, Tropical Asia and Australia).—Aponogeton distachyos and A. (Ouvirandra) fenestralis are grown in conservatories; the latter has lattice-like, perforated leaves.
Order 4. Najadaceæ. Only one genus Najas (about 10 species); annual fresh water plants with leaves in pairs and solitary, unisexual flowers. The ♂ flower is remarkable in having a terminal stamen, which has either 4 longitudinal loculi or 1 central one; on this account the stamen of Najas is considered by some authorities to be a stem and not a leaf-structure. The unilocular gynœceum and the single, erect, anatropous ovule are also terminal. Pollination takes place under the water.
Order 5. Alismaceæ. The regular, hypogynous flowers are in some species unisexual by the suppression of either andrœcium or gynœceum; they have a 6-merous perianth, generally differentiated into 3 sepals and 3 petals; generally 6 stamens in the outer whorl (by the division of the 3; Fig. [282]) and often several 3-merous whorls inside these, and 6–∞ free carpels arranged cyclically or spirally. Fruit a syncarp.—Marsh- or water-plants with radical leaves and long-stalked inflorescences.
A. Butomeæ. Follicles with many seeds, which are borne on nearly the whole of the inner surface of the cyclic carpels (as in Nymphæaceæ). Embryo straight.—Butomus (Flowering Rush, Fig. [282]), has an umbel (generally composed of 3 helicoid cymes). S 3, P 3, stamens 9 (6 + 3, i.e. the outer whorl doubled), G 3 + 3. B. umbellatus; creeping rhizome with triangular Iris-like leaves.—Hydrocleis. Limnocharis.
Fig. 282.—Diagram of Butomus: f bracteole.
B. Alismeæ. Fruit achenes. Latex common (in the intercellular spaces). The flowers are arranged most frequently in single or compound whorls. Embryo curved, horse-shoe shaped.—Alisma has S 3, P 3, A 6 (in 1 whorl, grouped in pairs, i.e. doubled in front of the sepals), and 1 whorl of 1-seeded achenes on a flat receptacle. The leaves are most frequently radicle, long-stalked; the lamina have curved longitudinal veins, and a richly branched venation. A. plantago.—Elisma (E. natans) has epitropous (turned inwards) ovules, whilst the ovules of Alisma, Sagittaria and others are apotropous (turned outwards).—Echinodorus (E. ranunculoides) has a convex receptacle, carpels many, united and capitate. Damasonium.—Sagittaria (Arrow-head) has monœcious flowers, several whorls of stamens and spirally-arranged achenes on a very convex receptacle. S. sagittifolia reproduces by tuberous buds formed at the end of long, submerged branches. The leaves, in deep and rapidly running water, are long and strap-shaped, but in the air arrow-shaped.