156 HENRY ALLAN GLEASON 



frequently descend gentle slopes and cross a narrow marsh in the 

 valley, without any change in the composition of the flora except 

 the addition of Ranunculus acris. There is, however, some 

 change in the relative abundance of the various species, and in the 

 size and luxuriance of the individuals. Similar areas of the blue 

 grass association occur throughout the uncultivated region, on 

 clay and on sand, on upland and on lowland, on forest clearings 

 and on abandoned farms, and all exhibit the same vegetational 

 structure. The association is therefore a product of human 

 activity and not of physical environment alone. 



Tables 5 and 6 illustrate the structure of the blue grass associ- 

 ation at two stages of its development in clearings. 



TABLE 5 

 Blue grass association, first year, 1915 



Pea pratensis 16 Epilobium angustifolium 2 



Phleum pratense 11 Erigeron canadensis 2 



Trifolium hybridum 9 Lepidium virginicum 2 



Polygonum convolvulus 5 Anaphalis marga itacea 



Trifolium pratense 5 Cirsium arvense , 



Capsella bursa-pastoris 4 Gnaphalium decurrens 



Trifolium repens 4 Hieracium aurantiacum 



Cerastixim vulgatum 3 Lappula echinata 



Chenopodium capitatum 3 Lychnis alba , 



Cynoglossum officinale 3 Sisymbrium officinale 



Rumex acetosella 3 



Twenty other species also occurred in the association, but not in the quadrats 

 counted. 



TABLE 6 



Blue grass association, third year, 1917 



Poa pratensis 96 Pteris aquilina 5 



Phleum pratense 21 Capsella bursa-pastoris 4 



Trifoliimi hybridum 13 Cirsium arvense 4 



Trifolium pratense 10 Arenaria serpyllifolia 3 



Rumex acetosella 9 Lychnis alba 3 



Trifolium repens 8 Rumex elongatus 2 



Agrostis alba 8 Chrysanthemum leucanthemum 1 



Poa compressa 6 Cynoglossum officinale 1 



Twenty other species also occurred in the association, but not in the quadrats 

 counted. 



A similar development of the same association takes place in 

 abandoned fields. The first season after cultivation the field 



