43° SECTIONAL TRANSACTIONS.— K. 



much of the vegetation to remain at the subclimax (or disclimax) stage. 

 Recolonisation takes place and some retrogression is also in progress. 



Glacial drift overlying the limestone has introduced many calcifugous 

 species and local topography permits the invasion of the limestone by 

 natives of siliceous soils. An intimate mixture of plants occurs on both 

 formations and species which differ widely in their soil requirements exist 

 in the same soil pocket. Some plants which are normally regarded as 

 calcifugous occur in the talus of the limestone screes. 



Certain species are restricted in their distribution, some are widely dis- 

 persed throughout a county and others are confined to one or two stations in 

 some areas and are abundant in similar formations only a few miles distant. 



Tuesday, September 7. 



Joint Discussion with Section M on Pasture problems (lo.o). 



Sir John Russell, O.B.E., F.R.S. — The general nature of the 

 problems. 



The problems of grass land differ in two ways from those of arable land : 

 (i) the crop is perennial and it may remain for an indefinite number of years ; 

 and (2) the crop is a mixture of several species, often 5 or 6, but sometimes 

 as many as 20 or 30. The perennial nature of the crop constitutes a great 

 agricultural advantage as it obviates the need for annual preparation of a 

 seed bed, one of the most costly items in arable farming, and it reduces to 

 a minimum the labour of cultivation and harvesting. The fact of the crop 

 being a mixture constitutes its chief scientific interest : grass land is one of 

 the richest sources of ecological material in the countryside. The different 

 species settle down to some sort of equilibrium, and for each set of condi- 

 tions there is a recognisable floral type which persists so long as the conditions 

 remain unchanged. Variation in weather conditions from year to year alter 

 the relative abundance of the different species, particularly on land receiving 

 no manure or only incomplete fertiliser. But the general floral type does 

 not change. Of the soil conditions, the supply of nutrients, the reaction, 

 water supply and air supply to the roots, and depth of soil, are all important 

 factors. On a poor soil nothing grows vigorously and every seedling has a 

 chance of life : the Rothamsted unmanured grass plots contain some forty 

 or fifty different species which have settled down to some kind of equili- 

 brium. Addition of plant food in the form of manure causes some of these 

 species to grow better than others and they crowd out their less vigorous 

 neighbours. The differently manured plots all carry distinctive fioras, the 

 outcome of this selective action. Acid conditions tend to eliminate legu- 

 minous plants : wet conditions to eliminate deep rooting grasses. 



The management of the grass land introduces some important new factors. 

 ^^'lld grass land speedily becomes very uneven owing to the action of ants 

 and moles, while the dead vegetation forms a mat which, if not removed by 

 earthworms, may become a layer of peat. On well-grazed land competition 

 for air and light disappear and the low growing wild white clover has a good 

 chance of life. Only those taller plants survive which can continue to shoot 

 up in spite of constant cutting. The successive new growths differ in some 

 respects from the continuous growth of an undisturbed plant and the ageing 

 continues, as shown by the production of more and more lignin as the 

 season advances. 



In grass land regularly laid in for hay the time of seeding becomes an 

 important factor in survival. These factors of agricultural management 



