Minnesota Plant Life. 467 



existing" in such plants, when connningied with the atmosphere 

 reduces its permeabiHty to heat, and thus the constant exhala- 

 tion of perfume from the body of a wormwood is to be regarded 

 as a device for tempering the heat of the sun. No doubt some 

 of these strongly scented substances are of value, too, as dis- 

 couragements to grazing animals. By all such adaptations in 

 the above-groimd portions of a plant it is clear that a reduced 

 transpiration is effected. The below-ground organs, such as 

 the roots or rootstocks of desert plants, are often important as 

 reservoirs of moisture. Some varieties, like the wdld morning- 

 glories, produce very large roots, and often the root system 

 of a desert plant is much greater in extent than the above- 

 ground portion. 



A number of classes of xerophytic plants exist, many of them 

 represented in Minnesota. While the state does not include 

 any desert regions, yet there are numerous tracts upon w-hich 

 xerophytic plants would find a habitat — for example, sandy 

 barrens, dry rocks, sand dunes and high sand beaches, high 

 prairies and meadows, or the dry branches of trees. Some of 

 the characters of these different classes may be discussed more 

 in detail. 



Rock plants. Rock vegetation is well defined and includes 

 a number of species that are not common elsewhere. A large 

 portion of it is made up of mosses and lichens ; and on dry 

 cliff's, exposed bowlders or crags, as well as upon ledges of 

 rock, a number of rock lichens and rock mosses are prolific. 

 These plants require but a small amount of water and are 

 often provided with several of the special adaptations against 

 transpiration. The common little black tufts of moss w^hich are 

 found upon bowlders, under ordinary conditions look perfectly 

 dry and shriveled, but if wet they quickly revive and their green 

 color becomes more apparent. Many of the lichens growing 

 upon rocks have the same shriveled, dry appearance betokening 

 adaptations affecting evaporation. In order to attach them- 

 selves to the bare surfaces of rocks, lichens secrete, as has al- 

 ready been stated in the chapter on lichens, certain acids able 

 to corrode limestone or quartz. The plants cling closely to the 

 corroded surface and they cannot possibly be detached with- 

 out destroying them. Some alg?e are capable of living on 



