246 THE JOURNAL OF INDIAN BOTANY. 



Successions within the Formations 



The three formations whose interrelations we have noted are 

 themselves advanced stages in the topographical succession. At many 

 places in and about this valley may be seen plant groups which do 

 not fit into any one of the three but which form an initial stage 

 leading to one of the three. As a topography tends to advance from a 

 rough tract to a plain, the hills being levelled by erosion and the 

 valleys filled by deposition, so plant societies tend to advance from 

 those of extreme dry or extreme wet situations to the climax form 

 which occupies a situation of moderate water supply. 



I. Xerarch Topographical Successions. 

 A topography may be dry because, — 



(a) It is so located that it receives a small amount of rainfall. 



(£) It is so located a3 to be subject to a large amount of evapo- 

 ration. 

 (c) The slope is such as to permit rapid drainage of water. 

 {d) The substratum is impervious to water. 



Much work could with profit be done toward the determination 

 of the local distribution of rainfall in the Himalayas. Certain places 

 such as Naini Tal have a high rainfall because a funnel-shaped valley 

 catches the incoming clouds and conveys them into a pocket surround- 

 ed by hills. Interior valleys receive less rainfall than the outer 

 slopes because the air has given up a large proportion of its water 

 vapor by the time it is forced over the crest of the outer range. It is 

 most interesting during the monsoon'to see how almost every day the 

 clouds pass through gaps in the outer ridge to move almost on a level 

 until they strike higher elevations further within the system. As the 

 hills became eroded and the valleys 'filled there would of course be 

 changes in the precipitation and with these we might expect vege- 

 tational changes. But the course of these changes is so slow that we 

 can get little direct evidence regarding them. 



A striking difference is observable whenever the north slope and 

 the south slope of a valley are compared. The former is well wooded, 

 while the latter is often an expanse of grassland with a little forest of 

 a xerophytic type in ravines and sheltered places. This difference 

 becomes more marked in the Himalayas as one goes westward to 

 sections having less rainfall. In the Chakrata region, for example, the 

 forest abruptly vanishes as the crest is reached or as the slope turns 

 from the north to a direction of greater exposure to the sun. The 



