EFFECT OF LIGHT ON UNDERGROWTH 159 



parts of the oak-birch wood, was not found in the oak-hazel. 

 Its presence has also been noted in oak woods and its absence in 

 oak-hazel. These differences may, no doubt, be explained by 

 the fact that the shade in an oak-hazel wood is greater than that 

 of an oak-birch. It is probably for this reason that in oak and 

 oak-birch woods bracken is found to be very plentiful, whilst in 

 oak-hazel it is somewhat rare. Bracken must have light, and 

 therefore flourishes best at the fringe of woods, or in those woods 

 in which the trees have not a close canopy. The thinning of 

 trees often favours its development, so, too, does the depredation 

 of rabbits on the shrubby undergrowth, for in both cases more 

 light is admitted. It is found that the herbaceous undergrowth 

 of oak woods to some extent depends on whether they are situated 

 in upland or lowland districts. The former are usually on drier 

 and more rocky soil with little humus, whilst the soil of the low- 

 land wood is damper and far richer in humus. The difference in 

 moisture affects the foliage of the trees ; it is denser, the shade 

 is greater, and the herbaceous undergrowth more prolific. Low- 

 land oak woods are rich in bulbous and early flowering species, 

 such as Anemones, Wood Sorrel, Early Orchis, Bluebells, Daffodils, 

 Woodruff, Wild Garlic. The upland wood has far fewer species, 

 about one-fourth the number found in the lower wood, and the 

 character of the vegetation is different ; it consists of plants 

 adapted to a drier soil, such as Foxglove, certain Potentillas, 

 Golden Rod, Hawkweeds and Grasses, such as Molinia ccerulea 

 and Deschampsia flexuosa, which are characteristic heath plants. 

 The same difference has been found in ash woods. In drier and 

 more exposed places, especially where the soil is shallow, the ash 

 is usually dwarfed ; but where the soil is deeper and damper the 

 ash reaches its normal height, and the herbaceous undergrowth 

 is considerable ; Dog's Mercury and Wood Garlic literally carpet 

 the ground, whilst Bluebells and Bracken are scarce. 



There is no doubt that the ash is an indigenous tree ; it was 

 probably the dominant tree of the historic forest of Mendip, 

 stretching along the Mendip hills above Wells and Cheddar, for 

 remnants of ash woods are still to be found in the copses which 

 spring up naturally in this area, as in Ebbor Gorge. It grows on 

 any good moist soil, provided it is deep enough. In winter it may 



