472 



Minnesota Plant Life. 



A great many prairie plants are annual but some of them 

 are shrubby and perennial. The shrubs are for the most part 

 low. This is no doubt in response to the moisture-condition, 

 because if they were high and loose, like the dogwoods and 

 willows of nuMster localities, their evaporation would be too 

 great. 



All of the formations that have been described are distinctly 

 xerophytic and in all of them the moisture-content of the soil 

 is comparatively small, while its temperature may be high. 



Pine forests. At this point it might be well to call atten- 

 tion again to the xerophytic characters of coniferous forests, 

 especially tamarack and spruce swamps, balsam thickets, and 

 river-blufT lines of junipers or red cedars. Probably the whole 

 pine forest may be regarded as xerophytic. The needle-shaped 

 leaves of the white and red pines are slow to transpire moisture, 

 and the pine forests reach their highest perfection along ridges 

 equivalent, in their topographical character, to the ridges of 

 rolling prairie. The moisture-content of such soil must be 

 below the average. Along with the pines a number of xero- 

 phytic mosses, such as the hairy-capped moss, of ferns, such 

 as the bracken ferns, and of flowering plants, including the 

 wintergreens, several heaths, and a variety of asters, are com- 

 mon. In such forests wand plants grow luxuriantly along with 

 the shrul)by underbrush. 



Air plants. When growing upon a sul)stratum in which 

 nutritive substances required by the plant are lacking or small 

 in amount, the surrounding atmosphere is drawn upon for the 

 food supply. Plants that habitually do this are termed air- 

 plants. The Spanish moss, tree-top orchids and perching ferns 

 of the tropics afford typical examples. In temperate regions 

 the perching vegetation is composed principally of lichens, 

 mosses, liverworts and alg;c. Much of the rock vegetation 

 previously mentioned may also be included in this class, and 

 here, too, may be classified the lichens and mosses that attach 

 themselves to the bark of trees, to fence rails, to the walls of 

 buildings or to absolutely arid soil. 



