THjH DISPKBSION of SMIBDS 337 



to the ground when the fruits are ripe, and have 

 no special means of becoming widely disseminated. 



414. There are certain structures and append- 

 ages of seeds and fruits, however, which favor 

 their dispersion. The most numerous types are 

 those which are carried by the wind. The thistle- 

 down and dandelion -balloons are familiar and per- 

 fect examples of wind -disseminated seeds. The 

 key-fruits (Obs. li.), and all the tribes of winged 

 seeds, are scattered by winds. 



415. In some cases entire plants, or large parts 

 of them, are carried long distances by the wind, 

 dropping their seeds by the way. ' A perfect ex- 

 ample of this mode of dissemination is the Russian 

 thistle, which is now invading great areas in the 

 West. The plant is easily detached from the earth 

 in the fall, and the dense, globular top — often 

 larger than a bushel basket — is caught up by the 

 winds and rolled mile after mile across the prairie. 



415a. The various "tumble-weeds" are dispersed in the same 

 way, of which familiar examples are the hair-grass (Panieum capil- 

 lare), white pigweed (Amarantus albus), and the cyclone plant or 

 cycloloma of the West. 



416. Many seeds are carried upon the coats or 

 feet of animals. The farmer knows how full of 

 seeds and "stick -tights" the sheeps' fleeces are if 

 the animals run in wild or weedy pastures. Many 

 strange plants spring up near carding factories, 



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