the Soils on zvhich they grow . 165 
Summary. 
1. A definite association exists between the species of weeds of arable 
lands and the soil on which they grow. The determining factor is the 
actual texture of the soil, and not so much the geological formation from 
which it is derived, except with soils overlying chalk. 
2 . The crop has very little influence on the weeds occurring except in 
the case of seed crops, which probably smother out a number of species 
which would normally occur. 
3. Certain weeds are definitely symptomatic of particular types of 
soil, though the majority of species are not so strictly circumscribed in 
distribution. Some species, also, are to be found indiscriminately on any 
soil. 
In conclusion I have to express my great obligation to Mr. Spencer 
Pickering, F.R.S., who obtained for me numerous introductions, without 
which the work would have been impossible. Also I desire to thank all 
those who so greatly facilitated the work by permitting me to visit them 
and by giving me much information and practical help. 
