Chapter III. 



Slime-Moulds and Blue-green Algae. 



Number of plants in the world. In the whole world there 

 are now living about 300,000 different kinds of plants, and it is 

 possible that nearly as large a number of forms are extinct. 

 The relics of past plant life in Minnesota are not very abundant, 

 but in the older rocks there are a few fossil sea-weeds deposited 

 on the mud flats of an ancient ocean which covered the region 

 now occupied by the land, and in the red sandstone which oc- 

 curs in the Minnesota valley in limited quantities, there have 

 been found imprints of leaves belonging to by-gone genera. 

 For example, in those days palm trees flourished in the state 

 and have left their replicas along with the remains of red-woods, 

 big-trees, cycads, magnolias, tulip-trees and other varieties, 

 which are not now found within hundreds of miles of Minnesota. 

 From old peat deposits and from soil masses lying under the 

 glacial clays, fragments of charred wood and vegetable debris 

 are sometimes exhumed. Such fossils often show that the dis- 

 tribution of plants before the glacial period was quite different 

 from that of to-day. 



Number and sorts of plants in Minnesota. At the present 

 time, of the 300,000 living species of plants, about 7,500 are 

 probably to be found growing without cultivation in Minnesota. 

 The figures are in the nature of an estimate, for by no means so 

 considerable a number has yet been discovered. But it must 

 be remembered that the larger proportion of these plants are 

 not the conspicuous objects which are usually in mind when the 

 word "plant" is used, but are rather the insignificant micro- 

 scopic forms, difficult to discover and often of such slight differ- 

 ence from each other that they would be recognized as distinct 

 varieties only by the most trained observers. 



