172 WEEDS OF FARM LAND 



may have often failed to take adequate account of the species. 

 It does not show a very pronounced objection to growing 

 among any crop, but " seeds," peas, and beans reduce it by 

 about one-half. The quantity occurring is reduced even more 

 than the frequency, for although fumitory is very often 

 dominant among most crops at certain periods of the year, it 

 was only once observed dominant among peas and beans. 

 Usually this plant gives very little trouble, as it is ephemeral 

 and the quantity is not often serious even when it is the chief 

 weed in a field. 



Association of Weeds with Cereal Crops. 



The cereals form a group of plants that collectively has 

 less direct influence upon the weed flora than any of the other 

 types of crop. The drills are placed some distance apart and 

 the habit of the young plant is such that the weed seeds have 

 a good opportunity of starting into growth under favourable 

 conditions. Later cultivation, where this is done, destroys 

 many of the weeds, but those that are present along the drills 

 themselves escape destruction. Consequently, the weed flora 

 of cereal crops remains fairly representative, except where 

 the crop is so exceptionally heavy that all undergrowth is 

 smothered out. Arable weeds occur with almost equal fre- 

 quency among cereals and among all crops added together, 

 and in most cases the figures practically coincide. A few 

 weeds are rather more frequent among cereals, as is shown in 

 the table on opposite page, but even with these the differences 

 are not great. Only two weeds, ribwort plantain (Plantago 

 lanceolata) and dove's-foot crane's-bill (Geranium molle), are 

 less frequent, but this is not due to discouragement by the 

 cereal crop, but to their introduction in large quantities in the 

 seeds of temporary leys. 



The weed floras associated with the individual cereal crops 

 show certain variations, but these differences are not at all 

 clearly marked. Every weed of any importance is found 

 amongst all the cereals, but some are more particularly encour- 

 aged or discouraged by one or other of them. Only two 

 weeds are so adversely affected by any cereal crop that they 

 practically disappear i.e. lady's mantle (Alchemilla arvensis) 

 and wall speedwell ( Veronica arvensis), which are hardly ever 

 seen among barley. In all other cases the association is less 

 definite. Some of the weeds can be grouped in two distinct 

 ways, according as they are more particularly associated with 



