13] VEGETATIONAL TYPES OF POLAR LANDS 395 



disappeared, and although the taller ones may still exceed 20 cm. 

 in height the sward is usually only 5-10 cm. high. Whereas it may 

 still be fairly dense, more often the ' heath ' is of scattered ground- 

 shrubs with intervening thin patches of Cetrarias, Alectorias, and 

 other Lichens as seen in Fig. 119. 



In the high-arctic belt heathy plants are entirely absent over 

 considerable areas, the tracts that are popularly spoken of as ' heaths ' 

 being usually dominated by Avens, Sedges, or even Lichens. How- 

 ever, Crowberry or Arctic Blueberry plants are to be found in some 

 regions, dominating limited heathy communities in unusually favour- 

 able situations, while Arctic Bell-heather is quite widespread, char- 

 acteristically forming a dark tract where the snow accumulates 

 sufficiently to form a good protective covering in winter (though 

 disappearing early in the growing-season). 



Arctic Fell- Fields and Barrens 



These are types in which the evident vegetation occupies less 

 than half of the area ; and whereas the two categories are scarcely 

 to be rigidly distinguished, it is usually those tracts that bear rela- 

 tively few and scattered plants that are referred to as ' barrens '. 

 Fell-fields typically have a surface of frost-shattered detrital material 

 including much finer ' soil ' and usually support fairly numerous 

 difi^erent species forming mixed communities, whereas barrens are 

 apt to be characterized by one prominent size of particle and a 

 single species of plant, such as Mountain Avens or Purple Saxifrage 

 {cf. Fig. 128). This is especially the case when they occupy the 

 most exposed situations. 



Where sufficient moisture is present these poorly-vegetated areas, 

 like some tundras, are commonly disturbed by all manner of frost- 

 heaving and allied efltects — such as solifluction on slopes and polygon- 

 formation on the flatter terrain. The solifluction is generally 

 manifest in streaks extending longitudinally downhill, adjacent 

 streaks being either of different material or accentuated by vegeta- 

 tion which cannot grow on the more dynamic parts. The ' polygons ' 

 are almost endlessly variable but very commonly have the form of 

 polygonal or circular areas \-2 metres in diameter and composed of 

 finely comminuted soil that is apt to be too dynamic to support 

 any plants at all, separated by narrow intervening tracts containing 

 most of the larger stones and often raised and vegetated or in other 

 cases barren (Fig. 120). Or the polygons may be separated by 



