PLANT MIGRATION. S7 



1. Seeds. ist. The buoyancy of seeds produced by the devel- 

 opment of so-called "wings" on the elm, maple, etc., the pappus 

 of the composites, and other hairy or woolly outgrowths, form 

 part of the seed or fruit which enable it to catch the wind. Small 

 size and lightness of many seeds also give them a certain amount 

 of buoyancy. 2d. The development of structures for grasping 

 hold upon other objects; for example, barbs and hooks on 

 seeds or fruits for clinging to animals. 3d. The use of seeds as 

 food by animals. 4th. Seeds which are capable of floating on 

 the water. 



2. Fruit. There are many fruits which are used for food by 

 animals and the seeds of which often pass through the body 

 uninjured, and thus often gain wide distribution. Exploding 

 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 fre- 

 quently 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 blown 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 



