38 



NATUEAL HISTOEY BULLETIN 



these species 32 produce seeds or fruits easily carried by the wind 

 and 33 have seeds or fruits which may be readily driven by wind 

 along the surface of snow, or with sand and dust. Seven species 

 are usually distributed by animals, the fruits of three being used 

 for food, and four producing hooks or spines. Many of the 

 seeds were probably also brought in on wagon-wheels, horses' 

 hoofs, etc. 



Once introduced, this flora has been able to hold its own be- 

 cause its members are essentially xerophytes. The larger trees 

 and shrubs which would make possible the advance of the forest 

 have been removed artificially; the smaller forest flora, being 

 mesophytic, can not exist in the strip thus exposed to wind and 

 sun ; and the prairie flora has become established simply because 

 its structural adaptations give it greater powers of resistance to 

 fluctuations in the relative humidity of the air. 



These structural adaptations are of the usual type, but per- 

 haps the most striking are shown in the leaf characters. The 

 leaves of the prairie plants usually have reduced surfaces, being 

 small or frequently variously cut, their texture is more or less 

 coriaceous, and they are frequently covered with hairs, scales or 

 spines. The difference in these characters between prairie and 

 forest plants is often illustrated in species of the same genus, as 

 is shown in figure 2, plate I. This figure illustrates the leaves 

 of species of eight genera. In each case (a) represents a leaf of 

 the forest species and (b) a leaf of the prairie species belonging 

 to the same genus. All but four of the species figured were ob- 

 tained on the prairie border or in the adjoining woods, but these 

 four species are found in nearby territory. Thus Viola pedati- 

 fida, while absent from the prairie border, is abundant on the 

 prairie north of the river. Its place along the road seems to be 

 taken by Viola pedata. Erigeron philadclphiciis and Phlox di- 

 varicata were not observed in the woods near the road, but both 

 occur in more remote portions of the same forest. Lobelia 

 syphilitica belongs to the swamp rather than the forest, but it 

 is found in wet places in the surrounding forest and is" also 

 introduced for comparison. 



The leaves of these species differ not only in form and size but 

 also in texture, those of the prairie being harsher and more 

 coriaceous. The usual differences in microscopic structure are 

 also strikingly shown, but these need not be discussed here. 



