544 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



where watercourses alone afford sufficient moisture for their growth. 

 Besides the more open and mature valleys, streams may cut narrower 

 gullies, ravines, gorges, and canyons, the steep and usually rocky sides 

 of which often afford habitats for plants that are not to be found 

 elsewhere in the vicinity. Consequently such plants may be useful 

 indicators of peculiar conditions — for example of rock crevices or 

 ' open ' soil and lack of competition, of a humid or shaded situation, 

 or sometimes of a particular type of substratum such as is afforded by 

 calcareous rock. Quite frequently valleys with the long axis lying 

 east and west have very different conditions on their north- and 

 south-facing slopes, which may be more clearly indicated by differ- 

 ences of flora and vegetation than by ordinary meteorological observa- 

 tions. These last are, moreover, tedious and often costly to make. 

 And though everybody knows that a south-facing bank in the boreal 

 regions tends to be sunnier and warmer than a north-facing one, it 

 may also be drier and so not necessarily preferable for all cultivational 

 purposes. Here again plants form useful indicators of the im- 

 mediately local conditions, Ferns for example being characteristic 

 of damp and shady banks whereas succulents occur on dry and 

 sunny ones. 



Of depositional features made by streams there are also many — 

 such as alluvial fans and cones, flood-plain deposits, channel bars, 

 deltas, and natural levees. Each of these has its tendency towards 

 supporting a characteristic vegetational type which usually varies 

 markedly in regions of different climate, particularly — although 

 certain plants, such as some Poplars, are widely characteristic, occur- 

 ring in similar situations in a considerable range of climates. These 

 stream depositional forms, being usually of fine material plentifully 

 supplied with plant nutrients, and commonly situated in sheltered 

 valleys, include some of the most vegetationally (and hence agri- 

 culturally) productive terrain. Being low-lying and often damp, 

 they may also produce fine pasturage. 



The residual features left by stream erosion are, from the point 

 of view of our present study, relatively minor ; though the vegetation 

 on their area may so bind the surface as to delay their formation, it 

 is usually meagre. Examples are divides and nionadnocks, and, in 

 part, those flat-topped erosion remnants known as mesas and huttes. 

 Even the residual forms of this category which are fairly extensive, 

 are liable to be rocky and exposed and consequently rather poorly 

 vegetated. 



Erosional features made by glaciers somewhat resemble those made 



