and maritime vegetation well developed we seek conditions entirely different, where amid shallow inlets and salt-marshes land and sea merge imperceptibly. Here the absence of higher plants from the areas below high water, as compared with their abundance above water-level, is a conspicuous feature. This is a noteworthy point, because if we assume that the presence of salt is the main factor which has prevented the land plants from spreading downwards, we are faced with the fact that the soil of the salt-marsh, where many such plants occur, may by evaporation of water become much more highly charged with salt than the sea itself. Yet the salt-marsh flora includes representatives of many Natural Orders, including some of the most highly specialized families--_Ranunculaceæ_ (_R. sceleratus_), _Cruciferæ_ (_Cochlearia_ spp.), _Caryophyllaceæ_ (_Alsine_), _Umbelliferæ_ (_Apium graveolens_, _Œnanthe Lachenalii_), _Compositæ_ (_Aster Tripolium_, _Artemisia maritima_), _Primulaceæ_ (_Glaux maritima_), _Plumbagineæ_ (_Statice_, _Limonium_). It seems clear that it is the assumption of the marine habit which is the stumbling-block, not the presence of salt. The Grass-wrack or _Zostera_, our only marine Seed Plant, comes of one of the oldest stocks of aquatic plants, and its nearest relatives have long been toying with the idea of a maritime habitat. The Order to which it belongs, the _Naiadaceæ_ or Pondweed family, from their worldwide range, their number, their variety, and their uniformly aquatic habit, may be set down as among the earliest Seed Plant colonists of lakes and rivers; some of them favour brackish water, while others besides the Grass-wrack have taken to marine life. Without going beyond the limits of our native _Naiadaceæ_ we can study the various stages, and form a picture of how the Grass-wrack migrated to the sea. First we have the numerous Pondweeds which grow in our lakes and rivers--plants with leaves broad and floating, or narrow and submerged, and inconspicuous flowers which rise above the water and are pollinated by the wind. Next we find several narrow-leaved Pondweeds which grow in brackish pools; and with them are some allies, the Tassel Pondweed (_Ruppia_) and Horned Pondweed (_Zannichellia_), with more reduced flowers and often a more nearly marine habitat, as they sometimes mix with Seaweeds on the open shores of estuaries; in these plants we find the stages of a most interesting return to the archaic method of water-pollination, so long discarded by the great mass of the Seed Plants. In the flower of _Ruppia_, which consists merely of two stamens and four carpels without corolla or calyx, the pollen is liberated under water, and, being light, rises to the surface; older flowers have already, by growth of the flower-stalk, reached the surface, and they become pollinated by the floating grains. In _Zannichellia_ the process is in general similar, save that the