Order 3. Pandanaceæ (Screw-pines) is another small order, forming a transition to the Araceæ. The woody, (apparently) dichotomous stem is supported by a large number of aerial roots, which sometimes entirely support it when the lower portion of the stem has decayed. The leaves are closely crowded together, and arranged on the branches in three rows, which are often obliquely displaced, with the formation of three spiral lines; they are, as in the Bromeliaceæ, amplexicaul, long, linear, the edge and lower midrib often provided with thorns. The ♂-flowers are borne in branched, the ♀ in unbranched spadices or capitula, which resemble those of Sparganium, but have no floral-leaves. Perianth absent. The drupes or berries unite into multiple fruits.—About 80 species in the islands of the Indian Ocean.—Pandanus, Freycinetia.—Fossils perhaps in the chalk of the Harz.
Order 4. Typhaceæ. The flowers are unisexual, monœcious, and borne on a cylindrical spike or globose capitulum; ♂ inflorescences above, the ♀ below. The perianth consists of a definite number of scales (Sparganium), or in its place numerous irregularly-arranged hairs are found (Typha); in the ♂-flower there are generally three stamens; the gynœceum is formed of 1–2 carpels with 1 prolonged style; 1 pendulous ovule. The seeds are furnished with a seed-cover, which is cast off on germination.—The few species (about 20) which belong to this order are marsh plants with creeping rhizome (and hence grow in clusters); the leaves on the aerial shoots are borne in two rows, entire, very long and linear.
Sparganium (Bur-reed). The flowers are borne in globose capitula; the perianth distinct, generally consisting of 3 small scales; pistil bicarpellate. Drupe, dry and woody. The stalk of the lower ♀ capitula is sometimes united with the main axis, and consequently the capitula are situated high above their subtending-leaf.
Typha (Bulrush, Reed-mace) has a long, cylindrical, brown spike, the lower portion bearing ♀-flowers, and the upper ♂-flowers, which is divided into joints by alternate leaves. The ♀-flowers have 1 carpel. The perianth is wanting, represented by a number of fine, irregularly-placed hairs; pistil unicarpellate. Fruit a nut.
The two genera, according to some, are related to the 2nd order. In both genera native species are found. The pollination is effected by the wind, and consequently the anthers project considerably, and the stigma is large and hairy. Typha is protandrous, Sparganium protogynous. The small, fine hairs surrounding the nut of Typha assist in its distribution by the wind.—Fossil Typhas in the Tertiary.
Order 5. Araceæ (Arums). The flowers are small, and always borne without bracts or bracteoles on an unbranched, often very fleshy spike, which is enclosed by a spathe, often petaloid and coloured (Fig. [301]). The fruit is a berry. Outer integument of the seed fleshy.—The leaves have generally sheath, stalk, and blade with distinctly reticulate venation; they are chiefly cordate or sagittate (Fig. [302]), seldom long with parallel venation as in the other Monocotyledons (Acorus, Fig. [300]). The Araceæ are quite glabrous, generally perennial herbs with tubers or rhizomes. Many have latex.—For the rest the structure of these plants varies; for example, while some have a perianth, in others it is wanting; in some the perianth-leaves are free, in others united; some have hermaphrodite flowers, but the majority unisexual (monœcious); some have free, others united stamens; the ovules are orthotropous, anatropous, or campylotropous, erect or pendulous; the ovary is 1–many-locular; some have seeds with endosperm, others without. In habit there are great differences. While some, e.g. Colocasia (Fig. [302]), have a thick, more or less upright stem, with leaf-scars, but not woody, others are climbers, epiphytic, and maintain themselves firmly by means of adventitious roots, on the stems and branches of trees, or even on steep rocks, e.g. Philodendron; the cordate, penninerved leaf is the most common (Fig. [302]), but various branched forms appear; the pedate leaves of Helicophyllum, Dracunculus, etc., are cymosely branched; the leaves of Monstera deliciosa, perforated by tearing, should be noticed (the vascular bundles while in the bud grow faster than the tissue between them, causing the latter to be torn, and the leaf perforated). With regard to the anatomical structure, the presence or absence of latex, raphides, resin-passages, groups of mucilage-cells should be noted. Engler makes use of these anatomical peculiarities for a scientific arrangement of the order.
A. Orontieæ, Calamus-group. ☿, hypogynous flowers of a completely formed monocotyledonous type (number in the whorls 2, 3, or 4).—Acorus (A. calamus, Sweet-flag) has a regular, 3-merous, pentacyclic flower (Fig. [300] C, D). They are marsh-plants, with creeping rhizome, triangular stem, and long, sword-like leaves (Fig. [300] A); the inflorescence is terminal, apparently lateral, being pushed to one side by the upright, sword-like spathe (Fig. [300] B).—Anthurium (Pr2+2, A2+2, G2); Pothos; Orontium (unilocular ovary with one ovule), etc.
Fig. 300.—Acorus calamus: A habit (much reduced); B inflorescence; C a flower; D diagram; E longitudinal section of an ovary; F an ovule.
