34 



Minnesota Plant Life. 



following spring. Then each membrane dissolves or breaks and 

 the fused-body extends itself into a new pond-scum thread, 

 becoming jointed as it grows and elaborating in each of the 

 joints the green ribbons by means of which it pro- 

 duces its food from water and carbonic-acid gas just 

 as did its parents the preceding season. By the 

 breaking in pieces of this filament in the way that 

 has been described and by the subsequent growth 

 of the pieces a large patch of scum, enough to 

 cover the surface of a small pool, will be produced 

 before the summer is at an end. 



Desmids. Related to the pond-scum are a large 

 number of tiny crescent-shaped and star-shaped 

 plants called desmids. These are often particularly 

 well-developed in the water of peat-bogs, so that if 

 one goes to the nearest tamarack swamp and 

 brings away a tumbler full of water which he has 

 squeezed out from among the peat-mosses, and sets 

 the tumbler in the window, within a few hours a 

 green film of desmids will be likely to form upon 

 the side of the glass turned toward the light. Like 

 the pond-scums these little plants have their breed- 

 ing habits and like them they are able to maintain 

 and distribute themselves throughout the water of 

 their pool. If in the autumn, the pool becomes 

 dry, the little eggs of the desmids lying among the 

 particles of soil may be caught up by the wind 

 and carried to distant pools where they continue 

 developing as before. 



Rolling algae. Not uncommon in Minnesota 

 is another bright-green alga which appears in quan- 

 tity in pools as green globules somewhat smaller 

 than pinheads. If placed in a saucer of water and 

 observed closely one of these green globules will 

 be seen to roll over and over in the water and make 

 its way from one side of the saucer to the other. 

 It does this because its surface is covered with tiny contractile 

 threads, which lash about in the liquid like so many little whip- 

 cords and roll the whole ball from one point to another as the 



FIG. 9. Portion 

 of a pond-scum 

 thread, show- 

 ing how it is 

 made up of 

 transparent- 

 walled cells 

 with a coiled 

 green ribbon 

 in each, much 

 magnified. 

 After Atkin- 

 son. 



