122 A NATURALIST IN THE GREAT LAKES REGION 



has increased in height. There are many other plants that 

 assist the cottonwoods to stay the onward-moving sands. 



Bunch grasses and the others mentioned above 

 are efficient binders. Their intertangled roots 

 serve to enmesh the sand and prevent its being 

 blown away. Sand cherry, the smooth and 

 glandular willows, bittersweet, horsetail, are 

 all forms that can stand the open sand areas 

 along with the cottonwood and when once 

 established help to hold the shifting sand. So 

 these 'form a part of the cottonwood association. 

 To say that these plants thrive here does not 

 mean that they live only in such barren places. 

 The cottonwood grows magnificently on rich 

 Fig. 87.— The soil; but here on the sands it is without com- 

 furry willow, Salix petitors. The red-osier dogwood is found 

 ^■^^ ^^^ ^' growing luxuriantly on the margins of the 



swamps, but it can endure the dunes and keep its head above 

 even rapidly accumulating sand (Fig. 89). 



k:% 



■='"*in' I ifcii 



»^':sss?-5| 



j^>>*^t:^ig. 



Fig. 88. — The cottonwood zone with fore-dune and storm beach in foreground 



The efforts of the plants to establish themselves and trans- 

 form the shifting sands into permanent soil would be in vain 

 were it not for the fact that the lake is constantly piling up new 

 dunes in front of those already formed. These, as they grow, 

 cut off the brunt of the wind from those in the rear so that the 

 conditions of life are less severe on these protected dunes, and 



