434 GENERAL MORPHOLOGY OF PLANTS 



shoots of many grasses for example, of ferns like the sensitive 

 fern, or bracken fern). Among those which extend their distri- 

 bution through roots, a striking example is that of certain species 

 of sumac. In New York state several species of sumac by their 

 seeds gain foothold in abandoned fields or in pastures. The 

 roots of these species spread many feet just underneath the sur- 

 face of the soil, and each year from the roots new shoots are 

 developed. The sumac often spreads from five to ten feet per 

 year in this way. 



616. Causes of plant migration. The plant has no choice 

 in the matter of migration such as man has. In the vast majority 

 of cases, and in all those where any great distance is covered, the 

 migration of the plant is a matter of accident so far as the plant 

 is concerned. The plants are moved from one place to another 

 almost wholly by physical agencies, by changes in climate, or by 

 the agency of man and other animals. 



The work of man in plant migration is carried out in two differ- 

 ent ways: by design and by accident. The history of agriculture 

 and horticulture abounds in accounts of the search for useful 

 plants, and their introduction and improvement. Many culti- 

 vated species have thus a much wider range and grow in far 

 greater numbers under the protection of man than could ever 

 have been possible had they been left entirely to natural causes. 

 In a number of cases the plants imported for cultivation "escape" 

 and establish themselves as " wild " plants, and thus become an 

 element in the flora of the region. A number of plants, especially 

 weeds, are introduced from one country to another by accident, 

 through commerce. Weed seeds may be accidentally present 

 with grains or other seeds of cultivated plants, or in the straw 

 used for packing. In other cases seeds are fraudulently adul- 

 terated by designing tradesmen; so that it has become necessary 

 for the government to establish a seed testing laboratory to 

 examine seeds and expose these fraudulent methods. 



617. Causes of plant migration initiated by plants them- 

 selves are found in: First: Fertility of species by which seed 

 results. Plants which have the power to develop large numbers 



