H4 ^ NATURALIST IN THE GREAT LAKES REGION 



more rapidly than the sand piles up, they would be completely 

 engulfed by the drifting sand. 



A dune, therefore, usually starts on some low-lying area near 

 the shore where the sand is sufficiently moist to allow many 

 cottonwood seeds to germinate, yet far enough from the storm 

 beach so the seedlings are not uprooted by the heavy waves. 

 Such a low, moist germination bed is known as a panne (Fig. 70). 

 That cottonwoods are the predominant growth in such a locality 

 is due to the fact that these poplars are the commonest trees in 



FIG. 69. Small dunes held by bunch grass and prostrate juniper, Jnniperus 

 horisontalis. 



the neighborhood, as will be explained shortly, and also because 

 their seeds are small, tufted with a silky pappus that insures 

 their ready transportation by the wind. Other plants that might 

 take kindly to such a place, like the red-osier dogwood, have 

 seeds that are not wind-blown. A wagon crossed a low area 

 near the lake west of Mineral Springs in the spring of 1912. The 

 ruts left in the moist sand facilitated the lodgment of poplar 

 seeds, and the next year a double row of seedlings was found and 

 photographed on the spot (Fig. 71). In two years' time a dune 

 was forming, as is seen from the photograph of the same spot 

 in 1915. In fact, in that time the sand had piled up somewhat 

 over two feet, and now it is shoulder-high. 



