244 MINNESOTA BOTANICAL STUDIES 



RedHeldia Hexuosa which has been accorded the honor of being the 

 very first pioneer in the reclamation of these sandy craters. Red- 

 iieldia may be the only plant in such places for many years, although 

 as a rule other early invaders, notably the leguminous species, 

 Psoralea lanceolata and Phaca longifolia, are to be found occupying 

 the same situations. Seedlings of numerous species have been 

 found in blow-outs in early spring. During the resting period the 

 disseminules of many species are blown into the deeper blow-outs 

 and may lodge there more or less permanently. With the resump- 

 tion of growing conditions in the spring many seeds germinate in 

 these spots and at such time the bottom of the blow-outs may be 

 covered with a veritable green sward composed of seedlings, most 

 of which however disappear before reaching much size because of 

 the approach of desiccating winds accompanied by the usual sand 

 movements. Seedlings of Psoralea lanceolata and Cristatella 

 jamesii are especially abundant at such times. Only the seedlings 

 of the most hardy pioneers are permanently enabled to resist the 

 fury of the approaching summer conditions, but these serve as 

 effective centers of propagation from which the vegetative wave 

 finally sweeps over the entire blow-out. 



I have seen numerous blow-outs in which the initial steps in the 

 development of the blow-out association were taken by Psoralea 

 lanceolata. However, in the most of the vast number of blow-outs 

 within the region RedHeldia flexuosa is the most abundant and con- 

 trolling species of blow-out pioneers. Hundreds of blow-outs occur 

 in which this is the only plant present. Still other grasses, notably 

 Muhlenhergia pungens and Eriocoma cuspidata toward the western 

 portion of the region, and Calamovilfa longifolia and Eragrostis 

 trichodes throughout, are of less importance during the initial 

 processes that eventually culminate in the stabilization of the blow- 

 out. Pound and Clements report {S7) that Calamovilfa longifolia 

 is regularly substituted for RedHeldia and Muhlenhergia in certain 

 localities of Antelope County. All these species are effective sand- 

 binders and, after RedHeldia or Psoralea has paved the way, they aid 

 greatly in the fixation of the substratum and in this manner facilitate 

 the establishment of numerous secondary species that follow in the 

 wake of the pioneers. 



The early phenomena in connection with the decadence of a 

 blow-out cover an interval of several years. For a number of years 

 after invasion RedHeldia extends its area by slowly pushing its 



