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.
(b) 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
