Type-Lesser Duckweed (Lemna minor).

A floating plant, consisting of minute green fronds bearing flowers in fissures of their margins.

The flowers are of extreme simplicity, consisting of a small membranous spathe, enclosing 2 stamens and a single carpel. Some regard each stamen as representing a distinct male, and the carpel a female flower.

Five species are native, one of which (L. polyrhiza) has been very rarely (? but once) observed in flower in Britain.

The flowers of the common species are frequently overlooked, from their minuteness.

Examine, with a lens, the tips of the slender roots of any Duckweed. They are covered by a highly developed modification of the cellular sheath, described at page 2.

63. Natural Order-Naiadeæ. The Pondweed Family. DISTRIBUTION.-Cosmopolitan aquatics, occurring both in fresh and salt water.-British genera 5, species 14. Floating or submerged plants. Perianth o or 4-leaved. Pistil

apocarpous.

Type-Floating Pondweed (Potamogeton natans). An aquatic herb, with floating oblong or elliptical op que petiolate stipulate leaves, and dense pedunculate spikes of minute flowers.

OBSERVE the sheathing axillary stipules of Pondweed : the tubular pollen-grains, destitute of an outer coat (extine), of Zostera, a plant with narrow, ribbon-like leaves, growing in the sea near low-water mark the exalbuminous seeds of the Family.

It is remarkable that most of the Orders of Monocotyledons of aquatic habit are destitute of albumen in their seeds.

Allied to the Pondweeds is the rare Lattice-leaf (Ouvirandra fenestralis) of Madagascar, the cellular tissue of the leaves of which is so reduced that open spaces occur between the veins, which form a delicate network resembling a "skeleton-leaf."

From the extreme importance to mankind, the number of species, and majestic aspect of the Palms, a short notice of the Order to which they belong (Palmaceae) is needful here, although none occur in Britain, and but a single_species, the Dwarf Fan Palm Chamærops humilis), is native in Europe.

Palms are perennial Monocotyledons, either arborescent, rising sometimes to a height of 100 or 200 feet, or acaulescent (having no apparent stem), with a short or prostrate rhizome. The stem is rarely branched, growing only by its terminal bud. A cross-section of the stem is well adapted to show the irregular arrangement of the isolated vascular bundles characteristic of Monocotyledons. The leaves form an immense crown, terminating the stem, and are often, singly, many feet in length. They are usually either of the radiate- or penni-veined type, though some are intermediate between the two forms. The flowers are

arranged upon a spadix, sometimes many feet in length, enclosed, at least at first, in a large spathe, similar to that of Arums, though often hard and woody. The flowers are small and sessile upon the branches of the spadix, consisting of a 6leaved or 6-lobed perianth, usually 6 stamens, and a tricarpellary pistil. Many of the species are diœcious or polygamous. The fruit is very variable both in structure and size. Compare, for example, that of the Cocoa-nut Palm, the Date Palm, and Vegetable-Ivory Palm. The seed is albuminous, and often solitary in the fruit when it is I-celled by abortion.

As in other Monocotyledons, no tap-root is produced when the seed germinates; so that, in order to secure stability to the tallgrowing stem, the cotyledon (one extremity of which remains in the seed to absorb the relatively abundant albumen) sometimes lengthens downwards into the soil to a considerable depth.

Amongst the more important species are :—

The Date Palm (Phænix dactylifera), invaluable to the desert tribes of Northern Africa, serving as the staple food both of themselves and of their cattle. The pericarp is fleshy and sweet. The minute embryo may be found on scraping the back of the seed with a knife, near the middle, embedded in a cavity in the horny albumen.

Coco-nut Palm (Cocos nucifera). The large seed is contained within a thick pericarp, the outer layers of which are fibrous, affording "coir fibre," used for mats, brushes, and the like. The inner layer of the pericarp (endocarp) is hard and bony, forming the shell, which the natives of tropical shores, where the Coco Palm grows, carve into various utensils and ornaments. The coat (testa) of the seed is very thin and not separable from the inner surface of the endocarp, unless the nut be dried near a fire for a few days, when the seed may be taken out entire. A crack may be carried round the shell in any direction by turning the nut in the left hand and applying smart blows with a hammer, two or three times all round, in the direction in which it is required to split. The minute embryo is concealed in a cavity immediately beneath one of the three " pores" near to the bottom of the shell. An oil used in candle- and soap-making is obtained from the seed by pressure: 156,000 hundredweights of it were imported in 1886. The juice which flows from wounds in the spadix of this and some other Palms is collected, fermented, and used as a beverage called arrack or toddy.

Coquilla nuts and Piaçaba fibre are afforded by a South American Palm, Attalea funifera. The endocarp is the part used by turners to make toys and trinkets. It is very hard and

takes a high polish. A fibre, resembling whale-bone, separates from the margins of old leaves.

Vegetable Ivory is the hard albumen of the seeds of Phytelebhas macrocarpa.

The Oil Palm (Elaïs guineensis) of West Africa affords Palmvil, used in the manufacture of soap and candles. The oil is obtained from the fruits. 873,923 hundredweights of the oil were imported in 1890.

Sago is obtained from the farinaceous cellular tissue of the trunk of species of Sagus, growing in the East Indian Islands. Palm sugar is prepared from the juice of Phenix and Borassus

in India.

Rattan canes are the long flexible stems of Indian species of Calamus, which resemble cordage stretched among the trees of their native Indian forests.

SUB-CLASS, Petaloideæ.

64. Natural Order-Alismaceæ.

The Alisma Family.

DISTRIBUTION.-A small but cosmopolitan aquatic

Order. British genera 6, species 9.

Perianth of 6 leaves. Pistil apocarpous. Ovary superior. Type-Water Plantain (Alisma Plantago).

same.

of Water Plaintain.

FIG. 167.-Embryo of same removed from the seed.

An erect aquatic perennial herb, with radical petiolate leaves, and unequal whorled peduncles forming a loose pyramidal panicle.

OBSERVE the difference between the submerged leaves reduced to linear petioles, and those which rise above the