503 
European plants here listed are to be found. This is the part 
of the state largely given to agriculture and a considerable 
portion is under irrigation. From 6.000 to 8.000 feet is the 
foothill zone having a climate favorable to agriculture but with 
little tillable land. Here, naturally, those introduced plants 
which flourish only in cultivated fields will be few in number, 
although roadside plants may be abundant. Above 8.000 and 
extending approximately to a height of 10.000 feet, is the mon- 
tane zone with practically no agriculture — only a few vegetable 
- or ornamental gardens with now and then a cultivated lawn. 
Here there is a further cutting down of opportunity for growth 
of introduced plants. In addition to lack of suitable ground- 
space, the factor of climate now enters, for summers are short 
and even in the hot months of July and August the mean 
temperature is only about 58° to 60° F. At higher altitudes, 
in the sub-alpine aud alpine zones, there are practically 
no introduced plants at all. Only a few straggling specimens 
here and there get above 10.000 feet. The following species 
seem to be the ones which reach the highest altitudes: Ramee 
acetosella, Polygonum aviculare, Thlaspi arvense, Bursa bursa-pastoris, 
Trifolium repens, Malva rotundifolia. 
Introduced plants east and west of the continental divide. So far 
as information is available it seems that the continental divide, 
usually 11.000 to 14.000 feet in altitude, has no influence on the 
distribution of introduced plants. All of the commonest species 
are present in both the eastern and western parts of the state, 
although, because of its larger population and more numerous 
railroads, the eastern part probably has a number of less-com- 
mon plants not yet introduced to the west. 
The weeds of Colorado not all of European origin. Although 
some of the European plants introduced into Colorado are abun- 
dant and persistent weeds, it should be noted that many of the 
commonest weeds of the state are true natives. Thus, in 
pastures and dooryards are to be found very commonly such 
plants as: Hordeum jubalum, Agropyron spp., umex salcifoltus, 
Euphorbia glyplosperma, Grindelia spp., and along roadsides Cleome 
