68 



Oenothera rhonibipetala (i) Brigeron rauwsus 



Verbena bracteosa Ambrosia psilostachya (9) 



Monarda punctata (4) 



It may be noticed that of the twenty species appearing in the 

 cjuadrats only four were interstitials, w'hile of the eleven more infre- 

 cjuent species not appearing in the quadrats seven were interstitials. 

 The relative frequency of the perennials is as 172 to 100. This illus- 

 trates and substantiates the general principle that the number of in- 

 dividuals of interstitial plants decreases as the density of the bunch- 

 grass increases. Although Ambrosia psilostachya has a greater 

 frequency than any of the perennials, it actually plays a very unim- 

 portant part in the consocies. Growing up straight and slender, 

 it is quite inconspicuous and really much less important than Aster 

 linariifolius. The high frequency of the latter species and of Cal- 

 lirJioe triangulata is also of interest 



2. Hanover area; up the hill (PI. Ill, Fig. i ) toward the plateau 

 at the southwest of the station just described. The consocies con- 

 tinues without interruption, but is somewhat different in appearance 

 (PI. II, Figs. I, 2). On the hillside the dominant species are the 

 same, but Panicum Scribnerianiim and Selaginella rupestris dis- 

 appear; the mats of Antennaria are sparse; the ground is not well 

 covered and does not have the loamy texture of the sand in the de- 

 pression. Bouteloua Jiirsuta becomes more abundant, corresponding 

 to the larger surface of open sand, and Linaria canadensis, an inter- 

 stitial, is common with it. The remaining secondary species are al- 

 most the same as in the valley. A transect up this hillside is shown 

 in Table II. The table shows that the change from the lower (left) 

 end of the transect to the upper is caused chiefly by the addition of 

 species as the space between the bunches becomes larger. In the 

 first half the average number of species per quadrat of 0.25 sq. m. 

 is 2.5, while in the last half it is increased to 4.35. 



