348 MINNESOTA BOTANICAL STUDIES 



definite time between periods favorable for growth, the conditions 

 which are too adverse for other forms of plant life permit at least a 

 certain amount of growth of lichen pioneers. "The crustaceous 

 lichens furnish most of the species which first gain a footing on the 

 rocks, and of these were found three or four species of Placodium, 

 a half dozen or more Lecanoras as well as a large number of 

 Biatoras, Lccidcas and BuclUas. Of the foliaceous lichens the 

 Umbilicarias are most characteristically rock pioneers." (Fink, 

 1899:221.) These might grow for hundreds of years before the 

 establishment of any other plant form in that area. Rock surfaces 

 may be found now which have not advanced beyond the crustose 

 lichen stage, but it is not possible to say how soon or how long 

 after the glacial period the invasion of such surfaces by the lichens 

 began. A long-continued growth of crustose lichens on a rock sur- 

 face, however, would tend to favor the invasion and establishment 

 of other forms of plant life. 



The formation of pools of water, even very small ones and of 

 short duration, in shallow depressions of the rock surface, would 

 provide a j^lace to which algae and aquatic i.nosses might be carried 

 and grow. This might happen repeatedly in the same place and 

 would in time result in the accumulation of humus, and with the 

 erosion of the rocks produce a small amount of soil in which other 

 forms of plants could grow. Even without the invasion of algae or 

 mosses, the accunuilation of water in pools w"Ould carry in particles 

 of rock that may have teen loosened as the result of weathering, 

 and in this way a small amount of soil be formed for the invasion 

 of plants. 



That algae and mosses may be carried to and live in pools of 

 water on a solid rock surface can not be doubted. MacMillan ( 1897 : 

 1017) mentions the occurrence on Windigo Island in Lake of the 

 Woods of Sphagnum cymhifoUum forming miniature peat bogs, of 

 two or three feet in extent and two and a half inches deep, upon 

 high wind-swept rocks. 



When pools of water are invaded by aquatics, it results in the 

 development of a miniature hydrarch succession. That this is true 

 is readily apparent from the instance above cited. When such pools 

 are of short duration, the hydrarch succession is so short that it 

 may be neglected. 



