Minnesota Plant Life. 



123 



Mosses are abundant, too, in very dry localities and are found 

 in little blackened tufts upon the bare surfaces of bowlders and 

 clifTs. Many of them cling to the bark of trees occupying posi- 

 tions like those frequented by lichens. Some are found regu- 

 larly at the bases of trees, the trunks of which serve as concen- 

 trators of moisture; for when the rains fall upon the twigs of 

 trees many of the drops are conducted along the branches to the 

 trunk of the tree and in this way the region around the base of 

 the trunk becomes more moist than the soil at a Httle distance. 

 Having taken advantage of this natural irrigation-system a va- 

 riety of mosses frequent the bases of tree-trunks. The kinds 

 which grow in such positions are in general different from those 

 growing on moist cliffs or on dry bowlders, or in swamps, or 

 mixed with grass along the road-side edge or in fields, or on 

 hillsides. 



Mosses and liverworts are distinguished from each other in 

 a variety of ways. They unite in having a young stage which 

 resembles the plant-body of the algae. Upon this buds are 

 formed that develop into the mature moss or liverwort plant. 

 Liverworts constitute a group of organisms more variable than 

 the mosses. They show more types of structure and have been 

 described as Nature's experiment-ground from which the higher 

 plants have originated. Two groups of higher plants are con- 

 ceived to be improvements of the liverwort stock. These are 

 the mosses (a terminal group) and the club-mosses or Christmas- 

 green plants which gave rise to the ferns and pines. Though 

 they are of somewhat different degree, as well as of different 

 kind of structure, the liverworts and mosses may well be con- 

 sidered together. Their habits and habitats are much the same. 

 In most instances their general appearance is similar, though 

 there are some liverworts which have the flat, prostrate, leaf- 

 like appearance of certain lichens, quite different from the leafy- 

 stemmed habit of the mosses. 



Breeding habits and life-histories. All of these plants pro- 

 duce true eggs and spermatozoids. After fecundation, the egg 

 develops into what is called a liverwort-fruit or moss-fruit. But 

 such fruits are really independent organisms, and here one 

 meets with that remarkable fact in the life. of plants technically 

 known as alternation of generations, — a phrase which means that 



