the Soils on which they grow, 159 
Gramineae. Agrostis stolonifera. Frequently dominant when found 
on gravel and chalk. Only occasional on clay and sand. 
Alopecuras agrestis. Distributed over different types of soil, but 
probably more typical of heavy clay ; scarce on chalk. May be dominant 
on either light or heavy soils. 
Poa annua. Distributed over all types of soil except chalk. Only 
dominant on heavy clay. 
Poa trivialis. Chiefly found on clay and chalk, but occasionally on 
rather sandy soil. Dominant both on chalk and sandy loam. 
Triticum repens. Chiefly found on the lighter types of soil, though 
recorded from clay. Sometimes dominant on sand, once on chalk. 
Equisetaceae. Equisetum arvense. Universally distributed and fre- 
quently associated with Coltsfoot. Occasionally dominant. 
An examination of the tabulated and classified lists of results reveals 
the fact that while some weeds are universal in distribution, occurring on all 
types of soil indiscriminately, other plants are definitely symptomatic, only 
occurring in certain habitats, being absent from all others. (N.B. — While 
a weed may be said to be absent from any particular soil, still it is possible 
for it to occur in that very habitat as a stray, though very rare or occasional.) 
A. Clay and Heavy Loam. 
These soils originate from different geological formations. 
(1) Clay with flints (e.g. Hitchin). 
(2) Gault (e.g. Harlington, Tingrith). 
(3) Oxford Clay (e.g. Flitwick, Houghton Conquest, Woburn Sands, 
and Ridgmount). 
Of these, very little data was obtained with regard to the clay with 
flints, and most of the results apply to the latter two. 
The weeds on the heavy lands were relatively few as regards the num- 
ber of species, especially in comparison with those on light and sandy soils, 
though the quantity of weed was equally abundant. It was found that with 
the solitary exception of Mentha arvensis , all the common clay weeds 
occurred on soils derived from both the Gault and Oxford Clay. Mentha 
arvensis was only found on the Gault and is very local, but it is quite 
possible that further search might discover it on the Oxford Clay as well. 
The weeds most characteristic of clay soils were — 
Bartsia Odontites 
Brassica Sinapis 
Chenopodium album 
Matricaria inodora 
Mentha arvensis 
Plantago major 
Poa trivialis 
Polygonum aviculare 
„ Convolvulus 
Ranunculus arvensis 
Senebiera Cononopus 
Veronica hederaefolia 
