502 
and roadsides. Probably the most persistent and harmful 
weed in Colorado. It extends to the montane zone. 
Lactuca virosa L. 
Plains area. 
Sonchus arvensis lL. 
Plains area; rare. 
Sonchus oleraceus lL. 
Plains area.. 
Sonchus asper (L.) All. 
Plains area and sometimes in foothill zone. 
Occurrence of the different species. Most of the European species 
in Colorado are roadside or pasture weeds, but some occur in 
lawns and in grain fields. The numerous irrigating canals 
throughout the cultivated area of the state, offer easy highways of 
migration for both xerophytic and mesophytic plants. Dry-soil | 
conditions obtain on the high ditch-banks, and moister soil is 
found closer to the running water or in Seepage areas at a 
distance. By far the largest number of species of introduced’ 
plants are known only from collections made in the irrigated area. 
This is partly because more collections have been made there 
than elsewhere, partly because there are more people and more - 
opportunities for seed-distribution, partly because the climate 
and soil-conditions are more suited to the plants which are 
introduced. Local areas. of distribution may be recognized for 
many European weeds. As an example may be cited the valley 
of Boulder Creek in the northern part of state, where Verbascum 
thapsus is everywhere abundant. This plant has been known in 
the locality for thirty years, yet it has not spread abundantly — 
to adjacent valleys a few miles distant. An interesting feature 
of weed distribution is the absence from Colorado of certain — 
European species which are abundant in the eastern and central 
United States, as Arctium lappa and Cannabis sativa. 
Altitudinal distribution of introduced plants. Altitudes in Colorado — 
vary from about 4.000 to 14.000 fe 
y et above sealevel. Up to 
5.500 or 6.000 feet, in what is 
the plains area, all of the — 
