Stapledon . — On the Plant Communities of Farm Land . 175 
having higher frequencies under roots, are much more luxuriant on a well- 
manured root field than they are under cereals ; examples are — 
Capsella Bursa-pastor is. 
Stellar ia media. 
Senecio vulgaris. 
Sonchus oleraceus. 
Anagallis arvensis. 
Chenopodium album. 
A triplex patula. 
Lamium purpureum. 
At high elevations, or elsewhere where the farming is poor, differences 
between the root and cereal communities are not so well marked ; and 
in this connexion it is interesting to note that a number of more 
or less typical corn weeds are usually either quite sporadic in roots or 
absolutely overrun the crop. They can be checked by the free use of the 
hoe, but if this is neglected they become even more abundant than under 
corn. Thus the normally greater abundance of Papaver Rhoeas , Brassica 
arvensis , Spergula arvensis, and Chrysanthemum segetum under corn than 
roots is due almost entirely to immunity from mechanical disturbance. 
(< b ) Vetches. 
The communities under this crop are usually meagre, for it is essen- 
tially a ‘ smothering 5 crop ; consequently the number of small ground 
annuals is inconsiderable. The presence of larger perennials will depend on 
the state of cleanliness of the ground. On the Cotswolds the following 
plants were usually found able to compete favourably with Vetches — 
Ranunculus arvensis , Galium Aparine , Convolvulus arvensis , and Polygonum 
Convolvulus ; and in dry seasons (e. g. 1911) certain tall plants introduced 
with the seeds are sometimes successful, e.g. Brassica alba, Lychnis Githago, 
Saponaria Vaccaria, Linum usitatissimum , and Cannabis sativa .* 
* Introduced with German and Russian samples of seed. 
(c) £ Seeds! 
The weed communities under seeds are more characteristic than those 
associated with other crops. Weeds under ‘seeds’ have to contend with 
the strong inter-specific competition set up by the growth of several 
million seedlings of plants having decidedly gregarious characteristics. 1 
These plants form a considerable tangle on the ground by the first autumn 
after sowing. It follows that the weed flora will be considerably affected 
by the degree of excellence with which the seeds ‘ take.’ This will depend 
1 e.g. from about 4-5 to 5*5 million for short leys and 18 million for long duration pastures. 
Observations on the Cotswolds and in Mid-Wales have shown that 1 million weed plants to the acre 
is an exceptionally high figure under cereals and roots. Oats give about 3 million and Barley 
2 million sown seedlings to the acre, so that the aggregate competition under Oats is between 
something under 4 million and Barley something under 3 million seedlings per acre, and there is 
also much more space between the cereal seedlings than between those of the grasses and clovers. 
