Monocotyledons. The higher phanerogams are distinguished from the Gymnosperms by their closed ovary as Angiosperms, and there fall into two main alliances, recognised by Ray and other predecessors of Linnaeus as monocotyledons and dicotyledons. The former are readily distinguished, as the name implies, by the single cotyledon of the embryonic plant, but also by a number of other important structural characters. Thus, the primary root (although it may develop strongly in germination, as in palms, lilies, maize, &c.) soon ceases to grow, and is replaced by lateral (adventitious) roots. The axis of the embryo also often dies away after producing lateral shoots, which may die in turn, and so on (e.g. sympodial rhizome of Solomon's Seal, tubers of orchids, &c.), but frequently also lengthens into the primary stem of full-grown plant (e.g. palms, aloe, maize, &c.); while young it grows (as in ferns) in the form of an inverted cone, each successive section being longer than the preceding until the adult size is reached, when the stem becomes cylindrical, tapering off only at the extreme point. There is thus usually no secondary growth in thickness (see, however, DRACENA). The leaves are seldom whorled, and the arrangement in two alternating rows is commonest. The leaf-bores are usually sheathing, and stipules are absent. The lamina is usually entire, the venation not ridged on the under side, and usually parallel. The fibro-vascular bundles of the stem do not anastomose nor form a ring, but are distributed throughout the whole stem, which thus does not exhibit the pith nor separable bark so familiar in many dicotyledons. The flower consists of alternating and isomorous whorls, outer and inner perianth, outer and inner stamens, and usually only one carpellary whorl; but from this type many characteristic specialisations arise. The endosperm is usually large and persistent, but some orders are exalbuminous.
The systematic study of the monocotyledons is most easily undertaken by clearly familiarising one's self with the Liliaceæ and their immediate allies, Amaryllidaceæ, Iridaceæ, &c.; and next by studying the progress of the floral specialisation, through Scitamineæ to its extreme in Orchidaceæ. Starting again from the lilies and their scarcely distinguishable allies, the rushes (Juncaceæ), we easily distinguish one series of degenerative (or more accurately vegetative) types, culminating in the sedges (Cyperaceæ) and grasses, commonly grouped as Glumifloræ. Another somewhat analogous line of change gives us the palms and Aroidæ, grouped at Spadicifloræ. The Helobie (including Juncagineæ, Alismaceæ, Hydrocharidaceæ) are also of special interest, as representing in some respects more primeval forms, and pointing back to a common ancestry with dicotyledons. See VEGETABLE KINGDOM, and minor special articles; also Goebel's, Van Tieghem's, or other text-books of botany. For systematic details, see Engler's Pflanzenfamilien or the Genera Plantarum.