CASES OF INVASION 451 



heat, or water, but also to their flowering and seeding at differ- 

 ent seasons. Many kinds of weeds nourish in grainfields, mak- 

 ing little growth until the grain is reaped, after which they 

 develop rapidly and flower and seed among the stubble. 



427. Invasion. Some of the ways in which plants are dis- 

 persed have already been described (Chapter xxxin). The result 

 of carrying seeds or other reproductive parts into new territory 

 is to cause an invasion of that area. If the invaded ground con- 

 tains no vegetation, the newcomers take full possession. Such a 

 case occurs when the bed of a newly drained lake or bayou, or 

 soil uncovered by landslides, or newly cooled material from vol- 

 canic eruptions is populated by vegetation brought in by natural 

 agencies. If the invading species encounter other occupants of 

 the region invaded, the new arrivals may simply share the ter- 

 ritory with its previous occupants. But if the immigrants are 

 much better adapted to the conditions of existence in the dis- 

 puted area than are its actual occupants, the intruders may drive 

 out all before them. 



428. Native species ousted by invaders. New Zealand and 

 the pampas of La Plata and Paraguay, in South America, have, 

 during the nineteenth century, furnished wonderful examples of 

 the spread of European species of plants over hundreds of thou- 

 sands of square miles of territory. The newcomers were more 

 vigorous, or in some way better adapted to get on in the world, 

 than the native plants which they encountered, and so managed 

 to crowed multitudes of the latter out of existence. 



In our own country a noteworthy case of the kind has 

 occurred so recently that it is of especial interest to Ameri- 

 can botanists. The so-called Russian thistle (Fig. 340), which 

 is merely a variety of the saltwort common along the Atlan- 

 tic coast, was first introduced into South Dakota in flaxseed 

 brought from Russia and planted in 1873 or 1874. In twenty 

 years from that time the plant had become generally distributed 

 as one of the commonest weeds over an area of about 25,000 

 square miles. 



