166 Sub-formation of the Chalk 



the beech, if the latter is growing in close canopy, for 

 their light demand is higher. The oak is extremely rare, 

 and is entirely absent from many beechwoods. Prunus 

 Avium is occasionally abundant. The jew (Taxus baccata) 

 is abundant in many beechwoods, this species forming a 

 distinct " layer " below the crowns of the dominant trees. 



Shrubs are practically absent in a normal beechwood, 

 for the light penetrating the canopy is insufficient to 

 permit of their growth. 



The ground vegetation is, for the same reason, very 

 scanty — often indeed quite absent. However, in places 

 where the foliage canopy is comparatively thin, Mercuri- 

 alis perennis is the typical dominant, often covering the 

 ground in continuous sheets (Plate XVI b), while Sanicula 

 euro'psea, Viola Riviniana, V. sylvestris, Fragaria vesca and 

 Gircma lutetiana are generally abundant and locally domi- 

 nant (Plate XVIII a) . Viola hirta, a pronounced calcicole, 

 is also abundant and characteristic. The orchids Ce-pha- 

 lanihera grandijiora, and the rarer Selleborine violacea 

 and H. atrorubens, are also characteristic, and Habeiiaria 

 virescens (chloroleuca) is frequent. In the humus the 

 colourless saprophytes Neottia Nidus-avis and Monotropa 

 Hypopitys are frequent. Mellehorus viridis, H. foetidus, 

 Atropa Belladonna, Daphne Laureola, and Riiscus acule- 

 atus are characteristic species of the more open spots in 

 or on the outskirts of beechwoods. 



Ashwood association (Fraxinetum excelsioris cal- 

 careum). 



The composition of this association does not differ 

 essentially from that already described (p. 147) as the 

 chief association of the formation of the older limestones, 

 though minor floristic diiferences of course exist. The 

 main interest of the association lies rather in its relation 

 to beechwood — the more typical woodland association of 

 the chalk. 



