30 Minnesota Plant Life. 



the water-flower and inquire for methods of exterminating it. 

 There is, however, no practicable way of destroying it, for if the 

 water should be poisoned enough to kill the water-flower other 

 living things, pond-lilies, bulrushes and bass would also disap- 

 pear. 



Structure of the water-flower. A microscopic examination 

 of the water-flower will show that each tiny jelly-ball consists 

 of a number of delicate algal threads intertwined and imbedded 

 in a common mass of jelly which they secrete. When a jelly- 

 ball becomes sufficiently large it commonly breaks in two and 

 the pieces continue growing as before. If the jelly be dissolved 

 hundreds of tiny coiled threads escape from it and multiply, 

 developing hundreds of new jelly-balls. In the autumn these 

 little creatures drop to the bottom of the lake, or remain in a 

 dormant condition frozen in the ice until the warmth of another 

 season stimulates them into renewed activities. 



Larger jelly-balls, in appearance very much like green plums, 

 but with the characteristic bubble of gas at the centre are often 

 found floating in ponds. These belong to another species of 

 blue-green algae but they are closely related to the water-flower 

 and their structure and life-habits are not essentially different. 

 Still another variety of blue-green algae is common as little 

 hemispherical lumps an eighth of an inch in diameter attached 

 upon the stems of bulrushes, just beneath the surface of the 

 water. Still other kinds form tufts and stringy masses several 

 inches in length floating near the surface of stagnant pools. A 

 curious form which has lost the blue-green color, and may be 

 classed also as a bacterium, is sometimes found growing in the 

 outlets of springs, where it resembles a mass of iron rust dis- 

 solved in water. The red color is not a deception for it is ac- 

 tually due to iron oxide extracted from the water by the micro- 

 scopic filaments of the plant. 



Rock-forming algae. Some of the blue-green algae have 

 the power of encrusting themselves with lime, and in watering- 

 troughs and tanks there sometimes occurs a calcareous forma- 

 tion reminding one of the deposit in old tea-kettles. Such a 

 crust is true limestone extracted from the water by the chemical 

 activities of the algae. Upon a large scale the blue-green algae 

 play their part in the formation of rock. The best place in 



