I 7 o HYDROPHYTES SECT, iv 



shore is flat ; only rarely are they met with at a depth of one metre below 

 the water-surface. In summer, when the water sinks and many stones 

 are uncovered, the incrustations crack and fall off; while in winter they 

 are rubbed off by ice. Thus it is that algae can contribute to the deposit of 

 lime in lakes. 



(6) Halo-nereid communities are those of salt-water. There are wide 

 distinctions between the floras of different seas ; but even on a single coast 

 there are many geographical features due to differences in the oecology of the 

 various species, as is briefly indicated in the succeeding paragraphs. 



The oecological distinctions depend largely upon differences in tempera- 

 ture, salinity, movements, and illumination of the water, as well as upon 

 fluctuations in these ; one important consideration is whether or no the 

 algae are periodically laid high and dry owing to tides, and another is the 

 height up the rocks to which breakers reach. 1 A prominent part is played by 

 differences in the soil (solid rock and its mineralogical nature, stones, rubble). 



The temperature of the sea is of importance. The most luxuriant ' forests ' 

 of Brown sea-weeds are developed in the coldest seas (frigid seas, North 

 Atlantic Ocean, coasts of Tierra del Fuego, southern point of Africa), possibly 

 because cold water is richest in air. In the southern seas named, some 

 individuals (Macrocystis, Lessonia) are hundreds of feet in length ; in the 

 North, species of Laminaria attain considerable dimensions for instance, 

 near Greenland Laminaria longicruris attains a length of 25 metres, and 

 Nereocystis 13!- metres in the Pacific Ocean. In tropical seas species are 

 smaller throughout. At Spitzbergen at the depth where vegetation is 

 richest the mean temperature of the water may not exceed at any season 

 of the year o C. 2 



The seasonal phases of species, according to Rosenvinge and others, 3 are 

 strongly marked, and a number of species present utterly different appearances 

 at different seasons. Some, including Chorda, Nereocystis, and a few other 

 Laminariaceae, are annuals ; but in other species larger or smaller parts 

 for instance, haptera and the inferior portions of the thallus perennate. 

 Rhodomela subfusca in the Baltic Sea during April and May bears a richly 

 branched shoot-system with reproductive organs which are subsequently 

 shed. Desmarestia aculeata likewise varies greatly in appearance with 

 the season. Some Delesseria sanguinea, for example fructify only in 

 winter. The cold water is richer than warm water in oxygen and carbon 

 dioxide and hence provides far better nutrition. 4 Kjellman's noteworthy 

 account of algal life in extremely northern seas has already received attention 

 in this work. 5 



Salinity of the water is another profoundly important factor affecting 

 the composition and appearance of vegetation. The farther we proceed 

 from the North Sea up the Baltic the less saline becomes the water, 6 and 

 the poorer and more reduced the vegetation. The Siberian region of the 

 Arctic Ocean is likewise poor in species, partly because the sea-bed is mainly 

 sand or clay, and partly because of the volume of fresh water pouring out 

 of Siberia. To fluctuations in salinity and temperature many species are 

 exceedingly sensitive. Some species endure a slight decrease in salinity, 

 others can adjust themselves to circumstances. 



Movements of the water (wave-violence, currents) and consequent increased 



1 Borgesen, 1905. 2 Kjellman, 1875. * Rosenvinge, 1898; seeOltmanns, 1905. 

 4 See p. 151. 5 See p. 22. 6 See p. 152. 



