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THE CHANGING GENERATIONS 



most animals spreading is active, caused by random wanderings in search 

 of food, mates, and shelter. In plants and some groups of animals the 

 mature organism cannot move about or has locomotor powers too limited 

 to carry it a significant distance. In these cases spreading is passive. It 

 is brought about by the transportation of seeds, spores, the resting stages 

 of small aquatic organisms, or the organisms themselves. The transport- 



Fig. 24.18. Devices for passive dispersal of fruits and seeds. A to F, carried by animals. 

 G to H, mechanically propelled. / to 0, wind-borne. A, cocklebur, Xanthium. B, stick- 

 tight, Desmodium. C, sticktight, Bidens. D, sandspur or sandbur, Cenchrus. E, burdock, 

 Arctium. F, porcupine, needle, or wire grass, Aristida. G, squirting cucumber, Ecbalium. 

 H, lupine, Lupinus. /, catalpa, Catalpa. J, milkweed, Asclepias. K, ash, Fraxinus. L, 

 dandelion, Taraxacum. M, maple, Acer. A'', linden or basswood, Tilia. 0, wafer ash or hop 

 tree, Ptelea. (Modified from Turtox chart, courtesy General Biological Supply House, Inc.) 



ing agencies include currents of air and water, and animal carriers. Many 

 or most of the species that depend upon passive spreading have a stage in 

 the life history especially adapted for transportation. Examples are 

 winged or fluffy wind-borne seeds, burrs that stick to the skins of animals, 

 the drifting coconut, and the minute swimming larvae of many sedentary 

 marine animals. 



In some animals there is definite behavior directed toward dispersal; 

 such behavior is called migration. It may be occasional, or sporadic, like 



