CLIMATIC FACTORS : THE AIR 



109 



appearing in the Dakotas have come from wind-borne spores pro- 

 duced as far away as Mexico. 246 



Seeds, fruits, and fragments of plants are effective -as dissem- 

 inules in proportion to the devices that facilitate their transport. 

 Wind dissemination is increased by the presence of winged struc- 

 tures, bladder-like protrusions, or plumose extensions of the 



FlG. 52. Approaching dust storm near Springfield, Colo. (1937), which 

 was typical of conditions in the "dust bowl" during the drought of the 1930's. 

 — U. S. Forest Service. 



surface (see Fig. 93). Seeds, because of their small size, are apt to 

 be carried farther than fruits, but for all, the kind of adaptation is 

 an important factor in transport. The perfection of the parachute- 

 like pappus is illustrated by the ubiquitous dandelion and related 

 composites of field and roadside. Many winged fruits do not travel 

 far because of their size, but often the wings (ash, elm, maple, 

 basswood) are sufficient to assure transport beyond the shading 

 and competitive effects of the parent tree. 



The transport of entire plants is well illustrated by the tumble- 

 weeds (Salsola, Cycloloma). These have but a single main root, 

 which, when broken at the ground surface, releases the spherical 

 plant to roll before the wind until caught, perhaps in some fence 

 corner. As it rolls, the seeds are gradually shed, sometimes miles 

 from the place of growth. 



The pioneers in a new habitat usually have effective means of 

 dissemination and an abundance of seed. The same is true of weeds 

 of cultivated fields and waste ground. The more common and 



