172 Sub-formation of the Ghalk 



In the above list ca is prefixed to the species which may fairly be 

 described as " ealcicole," at any rate in the chalk districts, though none 

 is absolutely confined to the chalk. An asterisk is prefixed to other 

 species which are specially abundant on the chalk. 



The chalk scrub often grows exceedingly thick and 

 then excludes almost all ground species; commonly, 

 however, it shelters many woodland species. Thus 

 Mercurialis perennis often occurs in sheets under its 

 shade. Among these woodland species are several of 

 the characteristic chalk orchids, such as Gephalanthera 

 longifolia, C. grandiflora, Helleborine atrorubens, H. vio- 

 lacea, Ophrys muscifera, Orchis purptirea, 0. militaris, 

 0. Simia,0. hircina, the last three of which are extremely 

 rare. The orchids of chalk grassland are given on p. 178. 



Trees of the beech, the wliitebeam, the yew and the 

 ash frequently occur in this scrub ; the relation to beech- 

 wood is often clear, though it differs somewhat from the 

 relation of scrub and woodland in the case of the forma- 

 tions of clays, loams and sands. This is owing to the fact 

 that while the shrub flora of calcareous soils is richer than 

 that of siliceous soils, the shade of the beechwood is so 

 deep as to exclude almost all shrubby undergrowth. 

 Hence while the scrub association of clay or sand is 



