MIGRATION AND DISTRIBUTION OF PLANTS 433 



uninjured, and thus often gain wide distribution, especially in the 

 case of migrating birds. Cedars are often seen growing along 

 fences where perching birds have dropped the seeds. Some 

 plants, like those of the mistletoe, are distributed by birds. Ex- 

 ploding fruits also bring about the dispersal of seed, as in the 

 vetches, the touch-me-not, the fruit of the witch-hazel; or of 

 spores, as in the sporangia of ferns, or of some of the fungi. 



3. Tumble-weeds. Several kinds of tumble-weeds are known, 

 some of which are popularly spoken of as " resurrection-plants," 

 especially certain species of club-mosses (Lycopodium). These, 

 as is well known, in dry weather curl their stems into a more or 

 less round, compact ball, and in so doing the roots are frequently 

 torn from their attachment to the soil, and the ball is rolled along 

 by the wind over plains to considerable distances. During the 

 rainy season these plants, which have retained their life in the 

 dry condition, expand and the roots take hold of the soil again. 

 Parts of plants, as the seed-bearing portion of certain grasses, 

 are often broken during heavy winds, and are blo\vn or rolled for 

 a considerable distance over the ground, thus providing for the 

 distribution of seed. 



4. Floating of broken branches. In the case of certain trees 

 or shrubs growing next to water the branches are often broken 

 by the wind and, floating to new places, sometimes aid in the 

 distribution of the species. 



5. Prostrate creeping plants or plants with a more rampant 

 habit migrate through a system of natural layering. In pros- 

 trate or creeping plants, like the strawberry, or trailing roses, the 

 stems take root here and there, and thus slowly and surely extend 

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

 habit, like the blackberries, or certain 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 walking. Example, the " walking- 

 fern " (fig. 302). 



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

 are a large number of well-known examples (the underground 



