Manual of the New Zealand Flora/Salviniaceæ
Order XCV. SALVINIACEÆ.
Fugacious annuals, of small size, floating in quiet waters. Stems simple or branched. Leaves small, often minute, apparently distichous, sessile or shortly petiolate, simple or lobed. Sporocarps or conceptacles on the under-surface of the stem, either clustered on the divisions of an altered submerged leaf, or in pairs in the axils of the leaves, globose or ovoid, membranous, indehiscent, of two kinds, both borne on the same plant; one kind containing a single or many macrosporangia, the other enclosing numerous microsporangia. Macrosporangia containing a single macrospore; microsporangia with numerous microspores.
Genera 2; species about 18, mainly tropical or subtropical. In germination the macrospore produces a rather large prothallium, which remains attached to it, and which bears one or more archegonia. The microspores each develop a rudimentary prothallium bearing a single antheridium containing spermatozoids, fertilisation taking place in the same way as in ferns. In the genus Salvinia, which is not found in New Zealand, the antheridia are formed while the microspores are contained within the microsporangium; but in Azolla the microspores escape in groups called massulæ, each with its proper membrane, and the antheridia are developed within the massulæ.
1. AZOLLA, Linn.
Floating water-plants. Stems copiously pinnately branched, emitting on the under-side numerous rootlets. Leaves densely imbricating, very minute, sessile, deeply and unequally 2-lobed. Sporocarps or conceptacles in pairs in the axils of the leaves on the under-surface of the stem, of two kinds: one kind larger, globose, enclosing numerous microsporangia, each of which contains numerous microspores arranged in separate groups or massulæ furnished with a membranous envelope; the other smaller, ovoid, containing a single macrosporangium within which is a solitary macrospore. Macrospores each crowned with few or many swimming-floats; massulæ of the microspores armed with simple or hooked bristles.
A small genus of 4 or 5 species, found in most tropical or warm temperate regions.
1. A. rubra, R. Br. Prodr. 167.—Floating, red or reddish-green, often gregarious and covering large sheets of water; the separate plants ½–1 in. long, orbicular or ovate or somewhat deltoid, copiously bipinnate. Leaves densely imbricating, about 1/20 in. long, 2-lobed, the lobes ovate, obtuse. Larger sporocarps globose, about 1/20 in. diam.; the massulæ of the microspores armed with copious hooked bristles. Smaller sporocarps hardly more than half the size, oblong; the solitary macrospore crowned with 3 swimming floats.—Hook. f. Fl. Nov. Zel. ii. 56; Handb. N.Z. Fl. 392; Benth. Fl. Austral. vii. 680; Bak. Fern Allies, 137.
North and South Islands, Stewart Island: Abundant in still waters throughout.
Also found in Australia and Tasmania, and very closely allied to the South American A. filiculoides, Lam., of which some authors consider it to be a variety.