BIOLOGICAL FACTORS 



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Vegetative structures may be effective in the same way. Aqua- 

 tic animals, such as muskrat, tear up rhizomes and bulbs, some of 

 which float free and establish new communities elsewhere. In this 

 connection, it is worth mentioning the importance of water as an 

 agent of dissemination, especially of floating propagules, even 

 though they do not retain their viability for long when saturated. 



Finally, the hooks, spines, and other devices characteristic of 

 many seeds and fruits insure their attachment to almost any ani- 

 mal contacting them and thus make possible their transport for 



FlG. 93. Structural modifications of seeds and fruits that facilitate dissem- 

 ination by wind or animals. (1) The parachute fruit of common dandelion 

 (Taraxacum) ; (2) winged fruit of dock (Rumex pulcher); (3) the silky- 

 haired seed of milkweed (Asclepias mexicana); spiny, hooked, and awned 

 fruits of (4) sandbur (Cenchrus paucifloriis) , (5) cocklebur (Xanthium 

 canadense), (6) red-stem filaree (Erodium cicutarium), (7) beggar's-tick 

 (Widens frondosa).— By permission, from Weed Control by Robbins, Crafts, 

 and Raynor, copyrighted 1942, McGraiv-Hill Book Company. 



