332 PLANT LIFE. 



Southern Russia and Western Asia. Barley seems to 

 have originated from " Hordeuin spontaneuin'' which is 

 found wild from Asia Minor to Baluchistan, as well as 

 in Arabia Petraea. 



Although the existing common rotation of crops is 

 exceedingly modern, only dating from lOO to 150 

 years back, the common weeds have been able to suit 

 their life-histories to the conditions ; and careful 

 examination will show that they vary both at different 

 times of the year and according to the crops. Potato, 

 Turnip, and Cornfields have all their characteristic 

 pests. In the hill-farms, which occupy most of the 

 country in the South of Scotland, the weeds are of a 

 much more humble and inconspicuous kind. Potentilla 

 toriJientilla, Galium saxatile^ Violets, and Polygala are 

 probably adapted to live amongst the grasses to which 

 their leaves, or leaf segments, bear a close resemblance. 

 Here the tendency is much more towards the formation 

 of a peatmoss, and Sphagnum may often be discovered. 

 On the very porous and thirsty soil of the South 

 Downs, it will be found that the weeds altogether differ 

 from those found in Scotland. 



The plants found upon the accumulations of black- 

 band shale, ballast heaps, and railway tracks form an 

 entirely peculiar and distinct series. The conditions of 

 these places are quite different from, for instance, those 

 prevailing on bare arable land, or even on ordinary 

 roadsides, or on the ground near human dwellings. 

 The cinders, gravel, or shale is unworked, virgin 

 material, and has at first but little organic matter, 

 consisting of rotted plant remains. It has no bacterial 

 population ; and, in addition, the ground is bare to the 

 sun's rays, and evaporation must be exceedingly vigor- 

 ous. Even some of the weeds of cultivated ground, 



