April 28, 192 1] 



NATURE 



273 



overlook the valleys from the north or the east 

 differ greatly from the anticlinal ones. The great 

 screes along the base of a northern or eastern 

 range are nearly, if not quite, bare; the rocky 

 stream-beds and the open slopes are sparingly 

 furnished with some of the more drought-resisting 

 members of the plant-formation on the valley- 

 floor below. The Little Pamir, however, is de- 

 scribed by Alcock as having grassy downs on 

 either" hand. The long axis of that Pamir runs 

 from west-south-west to east-north-east. We may 

 therefore conclude that the slope which looks 

 south also looks sufficiently east to escape ex- 

 treme desiccation, while the one which looks north 

 does not look sufficiently west to bring about that 

 condition. The western influence on this slope 

 may explain the absence from Duthie's Little 

 Pamir list of many of the species present in some 

 of the other Pamirs, in spite of the fact that 

 Alcock collected every plant he saw except a 

 rhubarb never met with in flower. In valleys 

 other than the Little Pamir the total number of 

 species recorded from mountain-slopes looking 

 south or west scarcely reaches a score, all of them 

 distinctly xerophytic in character. 



On slopes with an eastern, and especially with 

 a northern, aspect a relatively luxuriant flora, rich 

 in species of a more or less mesophytic character, 

 makes its appearance and constitutes a plant- 

 formation closely related to, and perhaps not really 

 distinct from, the mesophytic formation in the 

 haughs along the banks of the main stream below. 

 The two formations are, in fact, continuous 

 through the mesophytic vegetation that accom- 

 panies the streams, fed from snow-fields or small 

 glaciers, the broad channels of which open on the 

 valley-floor at right angles, and cross that floor in 

 order to join the river. Nearly four times as many 

 species are met with on mountain-slopes with a 

 northern exposure as may be found on those that 

 front the sun. The increase in amount of vegeta- 

 tion is even more marked than the increase in the 

 number of species. The poor and open furniture 

 of the sun-baked slopes looking south or west 

 gives place to a plant-covering usually closer, on 

 these moister slopes that face the north, than on 

 the open valley-floor. 



The relationship between the vegetation of a 

 flat Pamir and that of the containing slopes is 

 fully understood only if it be realised that the 

 valley-floor plant-formation is a "complex" of 

 at least three distinct plant-associations. When 

 this floor is quite horizontal all the species of the 

 formation may be intermingled ; but this condition 

 is rare. Usually the surface is undulating, and 

 more plants are to be found on the rises than in 

 the depressions. Some species in the depressions 

 grow equally freely on the rises ; a few prefer 

 the depressions ; one or two are confined to them. 

 On the rises the plants on the side facing north 

 or east differ from those on the side facing west 

 or south, and this arrangement is repeated with 

 every rise from end to end of a Pamir. Though 

 these slopes are never very pronounced, the 

 adjustment between the species concerned and 

 NO. 2687, VOL. 107] 



the conditions that affect them is so fine that,, 

 even when the inclination is too slight to be per- 

 ceptible to the eye or the muscular sense, the 

 alternating bands of species appropriate to the 

 anticlinal exposures demonstrate undulation of sur- 

 face, and reveal the effect due to the enjoyment of 

 a greater or less amount of heat and light, and of 

 a larger or smaller supply of moisture. 



Cushion-plants like Acantholimon diapensioides, 

 one of the commonest of High Pamir plants, may 

 occur on either aspect of a rise or in the depres- 

 sions between successive rises ; they may even be 

 met with occasionally on the screes. In spite of 

 this wide power of accommodation, Acantholimon 

 does not appear on slopes exposed to the north. 

 The very xerophytic Eurotia ceratoides, another 

 common and widespread species, is, however, 

 almost confined to the southern or western aspect 

 of the undulations ; this plant may occasionally 

 be found on the screes, and is perhaps the species 

 most characteristic of dry mountain-slopes facing 

 the south. These slopes, indeed, rather than the 

 valley-floor, might be looked upon as the dis- 

 tinctive home of Eurotia, were it not that the 

 genus invades from the valley-floor those moun- 

 tain-slopes that face the north. In many places 

 these latter slopes show faintly that alternation 

 of ridge and depression which is so marked a 

 feature of the valley-floor. The depressions oa 

 such a hillside provide a footing for vertical bands 

 of green vegetation composed wholly of meso- 

 phytic plants ; the ridges between, even when 

 barely perceptible to the eye, are marked by the 

 presence of sparsely scattered small tufts of 

 Eurotia. The grass Stipa orientalis, another 

 common High Pamir plant, grows freely on 

 either face of the undulations in the valley-floor,, 

 but avoids the intervening depressions. It is as 

 much at home on high slopes facing west or south 

 as is Eurotia ; often these two are the only plants 

 to be found on such dry slopes. 



Among the valley-floor plants that are confined 

 to the eastern or northern aspect of the undula- 

 tions is Trigonella Emodi, and it is on this account 

 that Paulsen has termed the vegetation of the 

 valley-floor the Trigonella-formation. It has, 

 however, to be noted that this species has not 

 been recorded from the Little Pamir, although 

 from Alcock 's account the vegetation of that 

 valley-floor is essentially the same as the vegeta- 

 tion of the other flat Pamirs. A species that occurs 

 only in the depressions on the valley-floor is 

 Arenaria Meyeri. This plant gives its name ta 

 the Arenaria-formation of Paulsen, a local plant- 

 formation which links the vegetation of the valley- 

 floor with that of the slopes exposed to the north. 

 The species most distinctive of these high moun- 

 tain-slopes with a northern aspect is Poa atten- 

 uata (Fig. 2), which Paulsen does not record from 

 any valley-floor, but which, it would appear from 

 what Alcock tells us, may be found in the Little 

 Pamir not only on the mountain-slopes to the 

 south, but also on the open surface of the valley, 

 and even on the downs to the north. On this 

 and on other mountain-grasses feed the herds of 



