STABLE AND MIGRATORY PLANT FORMATIONS JJ 



necessary that heath, moor, and marsh should be excluded 

 by edaphic factors, i.e. that souring or acidity of the soil 

 should be prevented. Good drainage is therefore essential 

 on the one hand, and sufficient bases to neutralise the acidity 

 induced by constant leaching, on the other. The former is 

 attained through soil-porosity or slope, the latter either by 

 periodic flushing of the surface with waters capable of pre- 

 venting acidity, or through the soil-layer being very thin and 

 resting on rock, which by weathering directly liberates 

 sufficient bases to neutralise acidity. The chalk and certain 

 limestones and basic-igneous rocks are the chief rocks which 

 may perform this function in this country, and owing to the 

 surface action of solvents tend to have smooth outlines, and thin 

 very finely divided residual soils on exposed elevations and 

 slopes. Exposed hills and bosses of these rocks are colonised 

 with difficulty by trees owing to the shallowness of the soil, but 

 afford all the conditions necessar}- for permanent grassland. 



The chalk grassland occupying the smooth, rounded 

 contours of the chalk downs in the south of England, is 

 believed to have existed from prehistoric times. Parts of 

 the tops of the downs, where a leached soil has accumulated, 

 show heathy patches (3), but the chalk is so pure that a soil of 

 this nature is generally absent, even on the gentler slopes. 

 Under present climatic conditions, humus fails to accumulate 

 under the grassland of the chalk downs. As in all pastures, 

 this may be partly due to constant grazing, but it certainly 

 is accentuated b\' edaphic conditions. During the colder, 

 wetter periods in the past, when the formation of moorland 

 peat was at its height, acid humus ma}- have accumulated 

 from the absence of drought and the inactivity of the bacteria 

 of deca\-, and the plant-associations of the chalk downs may 

 well have been very different from the present ones ; but with 

 the change of climate to a drier one with warmer summers, 

 the xerophytic grassland has found conditions peculiarly 

 favourable for its establishment. Under the present climate 

 neither acid nor mild humus accumulate, and the dwarfed 

 chalk-grassland flora has evolved under conditions promoting 

 recurrent drought and comparative soil-sterility, but without 

 the soil-acidity which defines the flora in similar sterile areas 

 on other rocks. 



