Water-plants, called "pond-weeds," and grow on both fresh and salt water. The marine species, Zostera marina, is dried and used for stuffing mattresses. [Illustration: Pond Weed.] 11. BUTOMACEÆ (Flowering Rushes). Of this order the Butomas umbellatus, a sort of rush, producing very handsome pink flowers in umbels, growing in ditches and by the sides of rivers, is the most characteristic member. [Illustration: Flowering Rush.] 12. ALISMACEÆ (Water Plantain). The two chief members of this order are the Alisma Plantago or Water Plantain, which grows in ditches, having its flowers in the form of panicles, and the Sagittaria sagittæfolia or Arrowhead. [Illustration: Water Plantain.] * * * * * The third great natural family of plants are the Exogenæ. They comprise all the trees and shrubs of the temperate and colder regions, together with many of the flowering plants. They are characterised by certain peculiarities which can be at once recognised, such as the twisted and branched form of the stem, the possession of bark, leaves having the veins covering them running in all directions and forming a network, and the seeds containing two cotyledons; the wood, moreover, is deposited in rings (figs. 16, 17, 18, 19), one of which is formed every year, by the new wood being produced on the outside of the old, and between it and the bark. This deposition takes place as follows: after the rains of winter and early spring have well saturated the earth with moisture, and the warmth of spring has begun to penetrate to the roots of the plants, a development of the points of each fibrile or radicle takes place, forming new spongioles; these, being formed of new porous cellular tissue, begin to absorb (by endosmose) the moisture of the earth, which entering at all these thousands of minute spongioles at once, collects and rises in the vessels of the trunk and branches, and arriving at the vessels forming a plexis on the surface of every leaf, begins to be changed by the action of the sun's rays, absorbing carbon and giving out oxygen from the carbonic acid always contained in the air. The sap which has thus risen is the juice of the earth in which the plant grows, containing several earthy salts and vegetable extract drawn from the manure or decaying vegetation contained in the mould, together with carbonic acid dissolved in the fluid; this carbonic acid is changed by the sun's rays as well as that which was contained in the air, and the carbon uniting with the watery part of the sap, forms the green substance before alluded to, called chlorophyll, which is the green colouring matter of all plants, and is the basis of the wood. The altered sap descends between the wood and the bark, and forms a deposit gradually, which at the end of the year is a complete ring of wood surrounding the wood of former years. This circulation of juices continues through the summer, until, the cold weather coming on and the