PLANT MIGRATION. 509 



stems take root he*-e and there, and thus slowly but surely extend 

 the area of occupation by the species. Plants of more rampant 

 habit, like the blackberries, or certain other roses, take root at 

 the tips of their branches, where they come in contact with the 

 ground, and new shoots develop at this point. This habit is 

 sometimes spoken of as the "walking" habit, and is well illus- 

 trated by the "walking-fern" (Fig. 493). 



6. Underground creeping stems or roots. Of this type there are 

 a large number of well-known examples, the underground shoots 

 of many grasses for example, of ferns like the sensitive fern, or 

 bracken-fern. Among those which extend their distribution 

 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 these roots new shoots are 

 developed. The sumac often spreads from 5 to 10 feet per year 

 in this way. 



977. Causes of plant migration. In general the causes of 

 plant migration may be grouped under two kinds of factors: 

 i st. What may be known as biotic factors. 2d. Physical and 

 climatic factors. 



1. Biotic factors. These factors are found both in the agency 

 of animals ,and plants themselves. Animals bring about plant 

 migration through the eating of fruits and seeds, or by dispersal 

 of seeds which cling to their bodies. Causes of plant migration 

 initiated by plants themselves are found in: ist. Fertility of 

 species by which seed results. Plants which have the power to 

 deve'op large numbers of fertile seeds with the best means for 

 seed distribution not only gain distribution through the seed, 

 but by the crowding of certain areas bring about pressures. 

 2d. The centrifugal habit of self-propagation by runners, by 

 layerings, or by the propagation of stems from separate roots. 

 3d. The factor of adaptation to environment, or acclimatization. 



2. Physical and climatic factors. Some of these have already 

 been mentioned under the head of structural characters favor- 



