BTOTTC FACTORS 285 



vegetation as moderate pasturing. Usually, however, mowed land 

 is also fertilized and, often, irrigated and seeded. This brings about 

 the anthropogenous rich meadows, which have such a surprisingly 

 uniform floristic appearance over large areas. All over central and a 

 large part of western Europe man has for a very long time controlled 

 the grassland according to uniform and widely accepted practice. The 

 result is a very uniform rich type of meadow with numerous species: 

 the Arrhenatheretum elatioris. 



The most extensive and richest hay meadows are not in the regions 

 of climax grasslands but in the regions of forests. Cutting and clearing 

 are usually followed by pasturing and only later by mowing for hay. 



The natural development of vegetation is permanently retarded by 

 regular mowing. The scythe exercises a rigid selection and kills off 

 all species, sooner or later, which do not ripen their seeds before the 

 first mowing or which have not enough regenerative power to shoot up 

 anew. 



The influence of manuring, irrigation, and reclamation upon 

 natural and semi-natural plant communities is so fully discussed in 

 agricultural literature that consideration of these topics here seems 

 unnecessary. 



Cultivated lands, such as vineyards, potato fields, grain fields, 

 truck gardens, etc., often exhibit quite characteristic "weed" commu- 

 nities, with numerous characteristic species along with the cultivated 

 plants. These communities reflect the specific ecology of the habitat. 

 We cannot accept Alechin's (1926, p. 43) view that everything of this 

 sort is accidental and artificial. These communities have a definite 

 geographic distribution and often show pronounced stratification. 

 They may exhibit very definite seasonal aspects; the composition of 

 the autumn aspect, the fallow period for grain and potato fields, is 

 very different in various associations but not less constant than it is 

 in so-called semi-cultivated communities. Allorge, Gaume, Luquet, 

 Kiihnholtz-Lordat, Tiixen, Koch, and Braun-Blanquet have dis- 

 tinguished and described a number of ecologically interesting associa- 

 tions of southern and central Europe which have been conditioned 

 by cultivation. 



Chevalier gives a comprehensive classification of the cultural plant 

 communities of the world, with the types of culture, in De Martonne's 

 "Handbuch der Geographic" (1925) to which the reader is referred. 



The ultimate aim of every reasonable attempt to control vegetation, 

 aside from purely aesthetic aims, consists in influencing with the least 

 possible effort the conditions of the habitat so as to give permanently 

 the greatest possible returns. But this goal can be approached only 



