272 



seemed to be smaller and more succulent than usual, but the stems 

 were thicker, and in the case of plants which are more or less pubes- 

 cent when young the pubescence was retained and frequently devel- 

 oped into villousness. All of these modifications were direct results 

 of the dryness of the summer. 



LIST OF THE SPECIES OF THE CAKIEE - XANTHIUM ASSOCIATION 



Dominant Species 

 Cakile cdcntula XantJiiuiii coinuiune 



Euphorbia polygonifolia 



Secondary Species 

 Conspcrinum hyssopifoliuiii Cycloloma atriplicifoVuiin 



Invading species (all of which are relatively scarce and are not 

 met with every year) 



Salsola kali fen it if alia Sali.v syrticola 



Popiihis dcltoidcs Potcntilla anscrina 



On the normal middle beach, only the first three of the dominant 

 species, mentioned above, are present. North of Winthrop Harbor, 

 however, where the ridges and swales are being washed away by the 

 waves, several other species are found on the middle beach. Their 

 presence is both accidental and temporary. The more frequent of 

 such plants are blue vervain {Verbena Jiastafa), mullen (Verbascuni 

 tJiapsus), sandbur (Cenchrus carolinianiis), strawberry (Fragaria 

 virginiana), white clover (Trifoliuui repens), smartweed {Polygo- 

 num persicaria), Poteniilla anserina, Polygonum acre, Panicmn capil- 

 lare, Acnida tuberculata subnuda, Polygonum lapafhifoliuui, horsetail 

 (Equisetuni arvense), and sand-bar willow {Sali.v longifolia). In 

 other places were the following additional species: blue grass {Poa 

 pratensis) , J uncus tenuis, Canada thistle {Cirsiuni arvense), Lythrum 

 alatuni, Radicula palustris, and red clover {Trifolium pratense). Al- 

 though these plants occur within the limits of the Cakilc-Xanthium 

 association, they do not properly belong to it for the following reasons. 

 Surrounding their roots, there is always more or less prairie humus, 

 which is sometimes only about the individual plants. In some places 

 there is a strip of prairie which, when undermined by the waves, 

 slides down on the middle beach, carrying with it whatever plants are 

 growing in it. Later, these strips are buried by a few centimeters of 

 drifted sand. The plants usually persist through the one season but 

 do not grow the next year. The burying process may keep up dur- 



