922 ECOLOGY 



rating the achene from the pappus, as in the lettuce and the dandelion, 

 or if the pappus hairs are branched, as in the thistle. 



In the milkweed (Asclepias) the seeds bear a crown of long, silky hairs at the 

 hilum end, which enables them to float in the air much as do the achenes of the 

 composite. Similar hairs facilitate dispersal in the willows and poplars, the cotton- 

 wood deriving thus its common name. Commercial cotton is derived from the 

 copious hairs that are attached to the seeds of the cotton plant (Gossypium); similar 

 cottony hairs are attached to the fruits of some anemones and of the cotton grass 

 (Eriophorum). 



A remarkable instance of wind dispersal is afforded by the tumble-weeds, 

 a class of plants that at maturity break off from the roots as a whole or 



FIG. 1218. A general view of mature plants of the winged pigweed (Cydoloma atripli- 

 cifolium), a representative tumbleweed; Gary, Ind. Photograph supplied by MEYERS. 



in part, whereupon they are tumbled along over the ground by the wind, 

 scattering seeds as they go (fig. 1218). Among such plants, which are 

 especially common on the prairies and plains, are the winged pigweed 

 (Cydoloma), the Russian thistle (Salsola Kali tenuifolia), and Amaran- 

 thus graecizans; these generally break off entire, but in the old witch 

 grass (Panicum capillare) and in some other plants, portions break off 

 and blow about alone or attached to other tumbleweeds. 



Dispersal by water. Water, though less effective than wind in the 

 number of seeds carried to places where they can germinate and grow, 

 is none the less a dispersal agent of great importance, particularly be- 

 cause it may carry disseminules for long distances. Sometimes the dis- 



