Stapledon.—On the Plant Communities of Farm Land . 1 79 
Conclusion. 
One object of the present paper has been to show that considerable 
advantages are to be gained by studying the weeds of arable land on 
a statistical basis, and in the light of the community as a whole. The 
application of this method immediately shows that, apart from anything 
else, species differ very much in their powers of colonization. Certain 
species, although they may be generally distributed over districts, are never 
numerically abundant ; others are, however, capable of forming considerable 
carpets on the ground. 
For instance, Ranunculus repens ^ Rmnex Acetosella , Spergida arvensis , 
and Veronica hederaefolia , and a number of other plants under congenial 
surroundings may have frequencies as high as 9. Euphorbia Helioscopia , 
Lapsana communis , and other plants, although they may grow under equally 
congenial surroundings, seldom attain to frequencies as high as 2. Thus 
when contrasting the behaviour of species under different conditions, it is 
necessary to have in mind their inherent capabilities as colonizers ; con- 
sequently the presence of such weeds as Spergida arvensis , Ranunculus 
repens , &c., in very small amount may, in certain cases, be just as or more 
significant than the complete absence of a species with a low habitual 
frequency. 1 It can be shown, furthermore, that a knowledge of the habitual 
frequencies of species makes it possible to gauge with some degree of accuracy 
their behaviour under unusual seasonal or other change. For instance, 
on the Cotswolds in the spring of 1912 (i.e. following the drought of 1911), 
a few species doubled and in some cases trebled their habitual frequencies, 
e. g. Lamium amplexicaule and Arenaria serpyllifolia , whilst Veronica 
hederaefolia everywhere attained to something approaching its maximum 
figure. Important, however, as it is to take frequencies into account, it is 
far more important to contrast whole communities, or at all events the 
chief contributing species of communities, rather than to interpret the 
influences of soil or of cropping in terms of the behaviour of certain ‘ index ’ 
plants ; this has been emphasized in the body of the paper, and was well 
exemplified when considering the flora of ‘ sour ’ soils. 
The results given would seem to show that the weed communities 
of arable land are (1) decidedly responsive to change in soil ; (2) are 
different near the altitudinal limits of cultivation, to what they are on the 
same soils at lower elevations ; this is, however, in part due to negligent 
husbandry ; (3) that they are also influenced by the crop under which they 
grow, but that this is largely due to the husbandry associated with the 
various crops. 
It has only been sought to compare the communities under Roots 
(including Swedes, Mangolds, and Potatoes), Cereals (including Wheat, 
1 The presence of species with high habitual frequencies in small amount only may be due to 
the activity of the hoe — a source of error always to be guarded against. 
N 2 
