Pool : THE VEGETATION OF THE SANDHILLS OF NEBRASKA 307 



abilities are invited. If the site is exposed to wind action the 

 almost invariable consequence is the immediate generation of blow- 

 out phenomena. But if wind action is not so severe, breaking 

 regularly leads to the early establishment of the various ruderal 

 combinations. These "weed stages" are most commonly repre- 

 sented by the species noted under the discussion of the ruderal 

 vegetation of the region. Through a series of changes which have 

 not been thoroughly studied the ruderals eventually yield their posi- 

 tion to the bunch-grasses. 



The spear-grass association may also be caused to revert to the 

 blow-out association after breaking, or such areas may be domi- 

 nated for a number of years by ruderal mixtures after which the 

 bunch-grasses invade and later the spear-grass again appears. 

 When valley land with either spear-grass or bunch-grass vegetation 

 is broken Hordeum jubatum often comes in in great abundance so 

 that such areas are very conspicuous. This same "weedy grass" 

 also invades, along with Cycloloma, Amaranthus graecizans, and 

 Salsola, areas of the wire-grass transition association after breaking. 



The lowland successional series begins with the aquatic pond- 

 weed association and follows more or less in correlation with the 

 developing humus complex through the water lily association to the 

 marsh formation. From this point we have traced the develop- 

 ment and structure of the bulrush-reed-grass and smartweed asso- 

 ciations with their common variations as they are correlated espe- 

 cially with a decrease in soil-moisture and a receding water-table. 

 As the substratum becomes progressively harder and drier we have 

 witnessed the development of the meadow formation with its rush- 

 sedge wet meadow and other associates, finally leading to the estab- 

 lishment of that extensive and valuable type which I have called the 

 hay meadow association. Rarely" is this~series interrupted through 

 the operation of the forces that so often affect the regular course of 

 upland successions. The hay meadow appears to mark the cul- 

 mination of this particular series, yet, while the evidence is not as 

 complete as it should be, conditions have been observed which 

 indicate that this type should be regarded as a "temporary climax." 



Certain evidences are prophetic of a still further development 

 from the typical meadow structure to that possessed by one of the 

 prairie-grass associations. Indications of such a possible transi- 

 tion have been observed where the hay meadows and certain prairie 

 associations have become intermingled. Modifications of the hay 



