172 Stapledon . — On the Plant Communities of Farm Land. 
It thus appears that in Mid-Wales there are only about twenty-five 
species which belong more essentially to the Wheat than to the no-Wheat 
zone, and that the majority of the abundantly occurring weeds are common 
to both zones. A number of the species which occur to only a slight 
extent at the higher elevations undoubtedly owe their origin to being 
introduced with the seed mixtures. Commonly occurring impurities have 
been marked with an asterisk in the above lists ; many of these, although 
normally weeds of young leys, may not appear till the land has been 
again ploughed, when they are manifest as isolated weeds in the corn, but 
being unsuited to high elevations remain small and stunted . 1 
The reason for the greater abundance of certain species at the higher 
than the lower elevations is probably not climatic. 
Thus the more frequent dominance of Spergtda arvensis , Chrysanthemum 
segetum , Ranunculus repens , and Rumex Acetosella is simply due to 
absence of the hoe ; whilst neglect in the use of the horse-hoe would 
account for the prominent position of Arrhenatherum tuberosum. Similarly 
Galeopsis Tetrahit 2 and G. versicolor , normally waste-place plants, are able 
to flourish amidst neglected root crops. The presence of such plants as 
Hypericum pulchrum , Vicia Cracca , Potentilla erecta , and otjiers marked 
with a dagger under section (d) is explained as follows : 
These plants do not occur to any extent under roots, but are often 
common under Oats. Oats always follow grass in the rotation at high 
elevations ; in many cases, however, the grass may have been down for 
eight to ten years, when it will have become very inferior, full of weeds, and 
of an indigenous character ; many of the indigenous plants are able to 
survive the process of ploughing and again appear under the first Oat crop, 
although not ordinarily speaking arable land weeds. Avena strigosa , 
grown as a cereal, occurs as an abundant weed in the following root crop. 
IV. The Communities under the several Crops in the Rotation. 
The weed flora found under different crops is partly influenced by the 
manner of growth and partly by the tillage operations connected with the 
husbandry of the particular crop. Under good farming arable land should 
be free from a number of familiar perennials, while a well-tended root crop 
should at all times be associated with but a meagre weed community ; 
under indifferent farming, however, e roots ’ present very favourable oppor- 
tunities for the spread of weeds, namely, a comparatively rich soil and 
abundance of bare ground. 
There can be no doubt also that arable land communities are affected 
throughout the whole rotation by impurities sown with the grasses and 
clovers ; this is particularly noticeable in districts where high farming 
1 e. g. Galium Aparine , Myosolis arvensis , and Silene Cucubalus. 
2 Seeds of this plant are frequently introduced with Oats. 
