318 



ELEMENTARY BIOLOGY 



expansions in the form of wings or tufts of hair that furnish 

 a large area in contact with the air (see Fig. 145). 



Seeds that have hooks, as the cocklebur and beggar-ticks, 

 attach themselves to the fur of passing animals and are carried 

 considerable distances from the parent plant (see Fig. 146). 



FIG. 146. Seeds scattered by passing animals 



/, beggar-ticks, or bur marigold (Bidens frondosd) ; 2, burdock (Lappula echinata); j, small- 

 flowered agrimony (Agrimonia patviflora); 4, carrot (Daucus c arotd) ; j, enchanter's night- 

 shade (Circaea lutetiand) ; 6, cocklebur (Xanthium canadensis) ; 7, bur grass (Cenchrus 

 tribuloides) ; 8, spike rush (Eleocharis watd) 



Seeds that are inclosed in edible fruits are often distributed 

 by being eaten by animals and then discharged from the intes- 

 tines without having suffered any injury. Cherries, black- 

 berries, and other small fruits are commonly distributed by 

 blackbirds, robins, thrushes, and other birds (see Fig. 147). 

 From the point of view of the species, there are three 

 factors in seed dispersal that are of fundamental importance : 

 (i) the number of seeds that are scattered ; (2) the distance to 

 which they are carried ; and (3) the final lodgment in a place 

 favorable to germination and later growth and development. 



