Chapter IV. 



Bright-green Algae. 



By far the greater numl.)er of Minnesota algae belong to this 

 group. 



Pond-scums. Probably the most easily recognized is the 

 familiar pond-scum, which is by many people regarded with 

 aversion and is supposed to be in some mysterious way con- 

 nected with the presence of frogs. If the slimy bright-green 

 scum be lifted from the pool, taken upon the fingers and closely 

 examined, it Avill be seen to consist of long delicate unbranched 

 hairs, not much thicker than a spider's web. As it dries in the 

 hand it curls and shrivels, but under the microscope the hairs are 

 beautiful objects. They are jointed and in each joint lie coiled 

 one or more green bands, like ribbons in a glass jar. By means 

 of the green bands the plant can construct starch out of the 

 carbonic-acid gas in the bubbles of air scattered near it through 

 the water, using also the water itself in the process. By the 

 breaking of the threads between their joints the plant abund- 

 antly propagates itself throughout the summer. 



When autumn arrives a very remarkable breeding-habit 

 comes into play. Two of the pond-scum threads extend them- 

 selves close together in the patch and from the joints of each, 

 little protuberances arise which become blended into tunnels, 

 uniting ordinarily the joints of one filament to the neighboring 

 joints of an adjacent filament. Through such tunnels the entire 

 Hving contents of one joint creep over and combine with the 

 living contents of a neighboring joint. The fused-body then 

 contracts like a sponge, expressing much of its sap and secreting 

 about itself a firm clear membrane. After such a process, 

 repeated throughout the patch of algae, the walls of the old 

 joints may slowly break or dissolve, and the hundreds or thou- 

 sands of oval fused-bodies, inclosed in their special membranes 

 drop to the bottom of the pool, where they lie dormant until the 



