70 MINNESOTA BOTANICAL STUDIES. 



ing end to end, and as they grow the filaments become encased 

 in calcareous sheaths. The feature of the plant which makes 

 it important in this discussion, however, is its habit of growing 

 in masses or colonies. The colony seems to start at some point 

 of attachment or on some object like a shell and to grow out- 

 ward radially in all directions, each filament independent of all 

 others and all precipitating calcium carbonate tubules. The 

 tubules are strong enough to serve as points of attachment for 

 other plants, and these add themselves to the little spheroid and 

 entangle particles of solid matter, which in turn are held by new 

 growths of the lime-precipitating Zonotrichia and thus a pebble 

 of greater or less size is formed, which, to the casual observer, 

 is in no wise different from an ordinary water-rounded pebble. 

 These algal calcareous pebbles show both radial and concentric 

 structure and might well be taken for concretions formed by 

 rolling some sticky substance over and over in the wet marl on 

 which they occur, but for the fact that a considerable number 

 of them show eccentric radial arrangement and that the shells 

 of accretion are likewise much thicker on one side than on the 

 other, and finally, because the side which rests on the bottom is 

 usually imperfect and much less compact than the others. The 

 pebbles are characteristically ellipsoidal in shape. The radial 

 lines, noticeable in cross-sections of the pebbles, are considered 

 by the writer to be formed by the growth of the filaments, while 

 the concentric lines probably represent periods of growth of the 

 plants, either seasonal or annual." Other forms than the Zono- 

 trichia were found in the pebbles. 



In June, 1901, Messrs. Freeman and Lyon, of the Botanical 

 Department of the University of Minnesota, found some cal- 

 careous pebbles in Clearwater lake, Wright county, Minnesota. 

 This lake covers an area of about four square miles and is 

 really two lakes connected by a narrow strait. The pebbles 

 were collected from the southern arm. They were found lying 

 in from four to ten feet of clear water on sand-bars, which rose 

 abruptly towards the surface and at their edges sloped almost 

 perpendicularly into deep water. 



These pebbles range in size from that of a small hickory nut 

 to two inches in diameter. Most of them are flattened, and, 

 though comparatively smooth in some cases, are often rough, 

 coagulated and wave-worn. All are more or less hollow. In 

 section they have a distinctly stratified appearance. The theory 



