BOG XEROPHYTES AND ACID SOILS. 113 



Transpiration is dependent not only upon insolation, air-temper- 

 ture, and relative humidity, but also upon the strength of the wind, 

 while root activity is determined chiefly by soil-temperature. The 

 open swamps and morasses of polar regions are at once the windiest 

 and possess the coldest soils of all habitats on the earth's surface. 

 The temperature of the soil remains very low for a long time after 

 snow has disappeared, owing to the gradual melting of the subter- 

 ranean ice, and even in midsummer the uppermost layers of the wet 

 soil are almost always constantly and considerably colder than in the 

 drier habitats. Even while the root-system is still partially frozen 

 at least, some species, such as Eriophorum vaginatum, begin to form 

 shoots and leaves, often to expose them for a long time to the drying 

 breath of the polar winds. In spite of excessive water-content and 

 relatively high humidity, swamp-plants are thus exposed to severe 

 drying-out, and many of them require protection against this danger. 



It is a well-known fact that in the high north many true swamp- 

 plants, such as Ledum, Betula nana, Andromeda, and Myrtillus 

 uliginosa, grow also in dry, sunny habitats, which without doubt 

 must frequently be very dry. Further south, Calluna and Empe- 

 trum exhibit a similar behavior, in that they grow in high moors and 

 at the same time in a dry, sandy soil, where the transpiration is 

 much greater. The explanation of this lies in the fact that the ever- 

 green shrubs of the swamp are annually exposed for a considerable 

 time to marked water-loss, when the ground is frozen and the snow 

 insufficient to protect them. The great majority of the plants 

 of peat-moors and swamps are therefore of a type that can with- 

 stand drying-out in the air and must often be exposed to it. In 

 some the leaves are scale-like or needle-like, stiff, and strongly cuti- 

 nized (Lycopodium, Diapensia, Andromeda hypnoides), or they tend 

 to be succulent (Saxifragra oppositifolia, Eutrema, Rhodiola). The 

 stomata are sunken or inclosed in hollows (Andromeda tetragona, 

 Empetrum) or covered with a dense layer of hairs below (Ledum, 

 Dryas octopetala, Potentilla nivea, P. multifida, Loiseleuria procum- 

 bens, Phyllodoce}. In other cases, the stomatal lower surface is cov- 

 ered with a thick coating of wax (Andromeda polifolia, V actinium 

 vitis-idcea, Salix glauca, and S. reticulata). Among the grass-like 

 plants are a large number of northern species that must be placed in 

 the steppe type by virtue of the rolling, hardness, and strong cutini- 

 zation of the leaves (Hierochloe alpina, Festuca ovina, Nardus, Carex 

 rupestris, C. pedata). In addition, there are many other sedges and 

 rushes of xerophytic appearance, such as Scirpus ccespitosus, Carex 

 dioeca, chordorrhiza, limosa, parallela, pauciflora, Juncus biglumis, 

 triglumis, and filiformis, as well as Equisetum fluviatile. It can not 

 be denied that some swamp-plants have no devices for reducing 

 transpiration. These are species with soft leaves, which neverthe- 



