918 



Relations of Plants to Subjacent Rocks, in the Chain of the Jura, fyc. 

 Translated from Mons. Jules Thurmann's 'Essai de Phyto- 

 staque applique a la chaine du Jura,' &c. 



The principal agents in determining the vegetation and the flora, 

 that is to say, the distribution of the species, are the following : — 

 Climate, depending chiefly on latitude and elevation ; then, climate 

 being equal, the physical characters of the subjacent rocks, with the 

 effects which result from them in reference to the hygroscopicity, the 

 quality, and the division of soils. 



The subjacent rocks, with reference to their mode of disintegration, 

 their power of absorption, and their permeability, are divisible essen- 

 tially into eugeogenous and dysgeogenous. 



The eugeogenous rocks produce an abundant detritus. When it is 

 of a clayey (pelique) nature, it makes humid stations that are often 

 inundated. When it is of a sandy (psammique) nature, it makes loose 

 soils that are almost always cool. When it is sandy and clayey ( pe- 

 lopsammique), that is to say, partaking of both natures, it makes sta- 

 tions with intermediate qualities. 



The dysgeogenous rocks produce a scanty detritus, sometimes sandy, 

 usually clayey, and always making stations more dry than those of 

 the eugeogenous rocks. 



To the eugeogenous subjacent rocks corresponds essentially the 

 presence of a group of plants which prefer humidity, or the hygrophiles. 

 To the subjacent eugeogenous clayey rocks correspond the hygrophiles 

 peliques, which particularly prefer cool stations. To the eugeogenous 

 sandy rocks correspond the hygrophiles psammiques, which particu- 

 larly prefer loose soils. 



To the dysgeogenous subjacent rocks correspond essentially a 

 group of species which prefer a certain degree dryness, or the 

 xerophiles. 



The hygrophiles peliques accommodate themselves on the dysgeo- 

 genous soils in certain cases, and are sometimes scattered over these. 

 The hygrophiles psammiques cannot usually live on dysgeogenous 

 soils, and are suddenly arrested on meeting them. The xerophiles 

 are scattered upon the eugeogenous soils in all points where these 

 soils afford a suitable dryness. 



The greatest contrasts in distribution are found among the hygro- 

 philes psammiques and the dysgeogenous soils. 



In proportion as we advance northward the hygrophiles accommo- 



