178 HYDROPHYTES sect, iv 



circumstances in species belonging to the limnaea-formation. The breadth 

 of the ribbon-hke leaf of Zostera marina is obviously adjusted to the depth 

 of water ; the shallower this is the narrower the leaf (variety, angusti- 

 folia) ; whereas in deeper water the plant becomes more vigorous and 

 broader-leaved. The narrow-leaved species, Zostera nana, also species of 

 Ruppia and Zannichellia, occur in shallow water not far from the 

 shore. 



In consequence of the far-stretching rhizomes a social mode of growth 

 results, so that dense, grass-green, submarine ' meadows ', often extending 

 for miles, are formed. 



The ftowers are very reduced, and inconspicuous ; ^ flowering takes 

 place on or under the water, and with its aid ; the pollen grains in species 

 with submerged flowers are filiform, as in Zostera and Cymodocea, or 

 linked together in long threads, as in Halophila,^ obviously in order that 

 they may be more easily captured by the long stigma, when conveyed 

 thither by the water, with which they agree in specific gravity. The stalks 

 of the female flowers of some species, including Enhalus and Ruppia 

 spirahs, are long and spirally coiled, and they contract after pollination. 

 The pollination of Enhalus is dependent upon the ebb-tide.^ 



GEOGRAPHICAL DISTRIBUTION 



In arctic seas this type of vegetation seems to be almost wanting, 

 possibly because the ice will not permit of its development. Several 

 associations of grass-wracks and the like may be distinguished.* In 

 north-temperate seas of Europe, Zostera marina and Z. nana occur, 

 while in the Mediterranean added to these are Cymodocea nodosa and 

 Posidonia oceanica. 



Zostera marina forms a shallow zone along coasts ; in Danish seas 

 its lower limit is eleven metres ^ ; the depth attained necessarily depends 

 upon intensity of the light, and thus upon clearness of the water. It 

 demands a soil that is to some extent protected. Other plants also are 

 partly confined to submarine meadows of this kind ; among such are 

 certain algae, some of which are epiphyllous, for example, diatoms, 

 Desmotrichum undulatum, species of Ectocarpus, Ceramium, Polysi- 

 phonia, often in great masses, while others, including Fucus, Laminaria, 

 Cladophora gracilis, and Fastigiaria furcellata,^ grow among the rhizomes. 

 This is the case particularly where the soil includes stones. In addition 

 there occur a series of more or less modified forms of various species 

 of algae, which have been brought hither by currents, and are detained 

 among the Zostera plants, where, remaining in a sterile condition, they 

 undergo more or less modification in their subsequent growth ; such 

 species are Ascophyllum nodosum var. scorpioides, Phyllophora Bangii 

 which is a metamorphosed form of P. rubens, also forms of Phyllophora 

 Brodiaei, Anfeltia phcata, Cladostephus verticillata, and others. In the 

 shallows of the sea off Schleswig, Cyanophyceae, as a microphyte-forma- 

 tion, also mingle with Zostera.' In Danish waters and the western 

 Baltic, close to land there is usually a shore-zone of Ruppia and Zostera 

 nana, while in deeper water there is one of Zostera marina. 



^ Schenck, 18866. ' Balfour, 1878; Theo. Holm, 1885. ' Svedelius, 1904. 

 * Ascherson, 1871. ' Ostenfeld, 1905, 1908 a. 



' Communicated by Rosen vinge. ' Warming, 1904, 1906. 



