Degeneration of Quercetum sessUiflorae 131 



rarely found. Now, as time goes on, the ground vegetation 

 of a wood tends to become quite closed ; and this simple 

 fact, in itself, is probably one of great importance in the 

 question of the rejuvenation or degeneration of forests. 

 Some foresters make use of their knowledge of the fact, 

 and go to great trouble in keeping the ground vegetation 

 open by removing the woodland " weeds." 



On many open hillsides, trees are more or less thinly 

 scattered. In some such cases, the ground vegetation is 

 of a grassy type ; in others, heathy undershrubs prevail. 

 It is believed that such examples of scrub are derived 

 from woodland, because every transition occurs between 

 woodland and scrub and because scrub frequently occurs 

 within the woodland limits of altitude. Further, some of 

 these scrubby places still retain the place-name "wood"; 

 and, in one or two cases, it is stated by the local in- 

 habitants that certain examples of scrub were woodland 

 in comparatively recent times. 



Whilst the majority of the examples of scrub that one 

 meets in the British Isles are apparently of this retro- 

 gressive nature, scrub of a progressive nature, that is, 

 scrub which is developing towards woodland, is sometimes 

 met with, particularly in deserted quarries. Progressive 

 scrub also occurs on fens and moors (see pp. 236, 

 250). 



Siliceous grassland associations. Grassland occurs 

 on the uncultivated slopes of the hills, where every 

 transition can be traced from woodland, through scrub, 

 to a clean turf of Nardus striata and Deschampsia 

 flexuosa. So far as one can judge, there are no soil 

 changes corresponding to these different associations. 



At the present time, grassland and scrub ascend to 

 higher altitudes than the woods; but it seems probable 

 that almost the whole of the land now occupied by grass- 

 land and scrub was once wooded. On some of the highest 

 summits, however, grassland now occurs on ground which 



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