178 Minnesota Plant Life. 



spore-producing generation of the plant is perennial, l)iit the 

 sexual ])lants die after they have performed their functions. 



Male and female plants. It is now possible to understand 

 the meaning of the curious sensitive appendages of the spores. 

 The spores when ejected are separated from each other into little 

 groups by the writhing of their appendages. The individual 

 spores are not, however, entirely isolated, and that degree of 

 moisture which is favorable for germination impels the append- 

 ages to pull the neighboring spores close together, so that when 

 they germinate, male and female plants shall not be too far apart 

 for the convenience of the swimming sperm. This is a very good 

 example of the extraordinary adaptive relations which come to 

 exist between sexual and spore-producing plants of the same 

 species. The appendages of the spores have seemingly no mean- 

 ing in the life-history of the spore-producing plant itself, but they 

 function in such a way that the task of the sperm-producing plant 

 is made easier and thus the development of fecundated eggs is 

 insured, for the perpetuation of successive generations. Upon 

 clay banks, where there are shade and moisture, one will often 

 find among the yoimg scouring-rushes or horse-tails some of 

 the tiny sexual plants looking very much like diminutive liver- 

 worts as they lie more or less prostrate upon the soil. 



Different sorts of horse-tails and scouring-rushes. The dif- 

 ferent kinds of horse-tails and scouring-rushes in Alinnesota are 

 distinguished by slight structural peculiarities that need not be 

 discussed in detail. The rigid, jointed. unbranclKnl forms, three 

 or four feet in height, which grow along shaded banks are per- 

 haps, in their tissues, the richest in silica or sand, and are the 

 ones which have particularly merited the name of scouring- 

 rushes. The very much branched variety which is such an abun- 

 dant weed in neglected fields, along roadsides, and in the edges 

 of woods, is a different S])ecies. A third species, in which the lat- 

 eral l)ranchcs curve downward in a characteristic way, is abun- 

 dant in northern woods and is named the forest horse-tail. Still 

 another kind is often found growing at the edges of ponds and 

 streams, now and then forming great patches in bays and occu- 

 pying the same general position that is ordinarily selected by 

 bulrushes. This, which may be termed the water horse-tail, is 

 commonly not very much branched although under certain 



