1004 MINNESOTA BOTANICAL STUDIES. 



water's edge in the former case and a zone of Spiraea in the 

 latter, upon the same island. Thus a distribution today of 

 plants upon the island shows the line of advance of the ancient 

 glacier quite as clearly as striae would. Such a fact indicates 

 well the long-standing nature of some of the causes regulating 

 plant-populations. 



In general the Scirpus barriers backed with Phragmites or 

 either of the two developed separately, will indicate a rather 

 flat nitrogenous type of bottom with the country rock well sil- 

 ted over. The Polygonum or Spiraea barriers indicate a bare 

 irregular rock bottom with humus laden crevices. Salix barri- 

 ers are indicative of drift collection. When one of these barri- 

 ers is fully established the area often becomes tenanted by 

 Potamogetons, upon which the force of the waves is modulated. 

 But large leaved plants do not collect in the barrier zone unless 

 it is broad enough completely to calm the "white caps" that 

 come in from the open water. Nor do small floating plants like 

 Lemma find it easy to remain in the interstices of the barrier, 

 so the subsidery vegetation is sometimes reduced to the vari- 

 ous algae, Cladophoras, Gloeocapsas, Scytonemas, etc. , that cling 

 to the stems of the dominant bulrushes or reed- grasses. 



ROCK SHORES. 



Shores of this type are almost invariably creviced, and when 

 crevices are formed they interfere with the regular zonal dis- 

 tribution of the plant population to such an extent that almost 

 all rocky shores will be better considered under the general 

 head of azonal formations. This is particularly true of talus 

 shores, for in the case of smooth rock in place there is usually 

 a well marked zonal distribution with reference to the water 

 line. Below high water mark on such shores the lichen flora 

 differs from that established above high water mark if the 

 shore be steep, rounded or shelving, but if it be flat and a trifle 

 above the water line, as sometimes, there is not only zonal dis- 

 tribution of the ordinary type to be observed, but very often 

 too that biological distribution from a common center that is 

 perhaps best indicated in the "fairy-ring" mushrooms of mead- 

 ows and fields. On such flat rocks circles are marked out by 

 the growth of rock-lichens, and these overlap and interlock 

 in an interesting fashion. A general view of lithophytic vege- 

 tation may be postponed until the azonal shores are considered 

 and here only a few points regarding the ordinary zonal distri- 

 bution need be remarked upon. 



