Minnesota Plant Life. 



443 



Plankton. The first class of hydrophytic vegetation is that 

 known under the technical term of plankton vegetation. By 

 this is meant the passive, free-floating vegetation, not rooted 

 or attached to the soil in any way. To this type, no doubt, 

 belong the earliest forms of life that appeared upon the crust 

 of the earth. Here are to be classified many algae such as 

 the familiar water-flower of Minnesota lakes, which, as will be 

 remembered, is a type of blue-green algae. A number of Min- 

 nesota varieties belong to the algal plankton, for here are in- 

 cluded not only the water-flower, but such forms as the sphere- 

 alga, the pond-scum, the desmids and diatoms, the rolling alga 

 and a number of others. Some of these plants are characterized 

 by the production, between their filaments, of interlocked gas 

 bubbles, by which they are enabled to float at or near the sur- 

 face of the water, and thus to receive the sunlight. In many 

 of them there are formed reproductive bodies provided with 

 swimming lashes, and by the aid of such little motile cells the 

 plant may be distributed throughout the water of a lake or pond. 



Another group of plankton vegetation is constituted by the 

 bacteria that live in water. A great many different sorts are 

 known ; not only from fresh water, but from the sea. Any drop 

 from a Minnesota lake or river, if carefully examined by the 

 proper methods, would be found to contain great quantities of 

 the bacterial organisms. 



Still another group of plankton vegetation is what may be 

 called derived, or secondary plankton. This group is composed 

 of passive, free-floating plants which are, from their structure, 

 evidently derived from ancestors that were rooted. To this divi- 

 sion belong the little water fern Azolla, the duckweeds, form- 

 ing such abundant scums on stagnant pools, and the bladder- 

 worts, together with some varieties of liverworts, such as the 

 swimming Riccia and the floating Riccia. A number of pecul- 

 iar adaptations exist in plants of the derived plankton. Often 

 in their bodies there are air chambers by means of which they 

 float. These may be seen in cross sections of duckweed plants 

 or Riccias. Sometimes the air chambers are developed as spe- 

 cial pouches or bladders, as in bladderworts, where they also 

 fulfill another function that of capturing small animals for 

 food. If the plant floats on the surface of the water, with its 



