562 REPOBT OF STATE GEOLOGIST. 



sea-rocket (Corispemnum hyssopifolium) and prickly pear (Opuntia 

 humifusa) suggest themselves as familiar examples of this type of 

 plant. Still more do the stunted forms, the reduced leaves and the 

 surface hairs or epidermal thickenings of most plants in such region* 

 tell the story of the arid soil. 



III. Intermediate plants (Mesopliytes) or those adapted to inter- 

 mediate conditions of both soil and air moisture. Such plants include 

 the great majority of our forms, since these conditions prevail through- 

 out the entire State. The plants of use to man, as well as the weeds, 

 belong to this group, since plants to be either of high economic value 

 or serious injury to man must be such as are adapted to these medium, 

 or average conditions. Mesophyte conditions are, indeed, agricultural 

 conditions. If a farmer has a swamp tract (hydrophyte conditions) 

 which he desires to cultivate, he drains it, that is he strives to bring it 

 to mesophyte conditions. If it is wished to redeem an arid region (xero- 

 phyte condition) irrigation is employed that the mesophytic condition 

 may be reached. This mesophytic or intermediate flora stands there- 

 fore as the index of the agricultural fitness of the land so far, at least, 

 as the water content, of the soil is concerned. 



Sub-divisions under each of these great groupings, based upon other 

 factors influencing the distribution of plants, can be made, showing 

 how perfectly the indigenous flora of a region stands as the sign of its 

 agricultural capacity. While all plants tell the story, none tell it so 

 plainly as the trees, whi'ch represent the ultimate possibility in a vege- 

 tative way, of soil and sun and air and centuries of time. It is impos- 

 sible, with the data at hand, to more than indicate some of the more 

 important of these sub-divisions, such as "clay-soil societies," "lime- 

 stone soil societies/' "sand plants/' "cliff plants" and "swamp plant" 

 societies. It must be remembered, however, that no one plant stand- 

 ing alone is characteristic of any given set of conditions, although a 

 group of a half dozen or more species can usually be selected which 

 will, as a rule, not be found associated under any other set of condi- 

 tions. It is the ability to distinguish these characteristic groupings, 

 which enables us to read the message of the plants. 



It will be seen from the above, that it is quite possible to construct 

 a map of the State showing the distribution of these three great groups 

 with a fair degree of precision. Such a map would be a phytogeo- 

 graphic map. In the past four years there is a growing inclination to 

 call the study of plant distribution, pliytogeography. 



The large Hydrophyte areas would be found in the great Kankakee 

 marshes, about the innumerable small lakes of northern Indiana, in 



