42 Minnesota Plant Life, 



which upon being broken have a moist bluish-green interior or 

 are hollow, he will doubtless have discovered a growth of rock- 

 forming algae. 



Algae the oldest kinds of plants. The great group of 

 algae is of peculiar interest to students of nature because it 

 includes the oldest types of plants living upon the earth. There 

 is reason to suppose that life originated in the sea, and that all 

 the terrestrial forms are descendants of those which in distant 

 epochs learned to leave the ocean and establish themselves upon 

 the land. One reason for supposing this is that in distant peri- 

 ods of the earth's history there was very little land in existence, 

 and almost the whole surface of the globe was covered by the 

 waters of the ocean. No doubt, at first, before the ocean had 

 cooled, when the world was still young, warm-water algae, 

 among which the rock builders are so prominent, came into 

 being and began their work perhaps among the very first living 

 creatures of all the hosts that now exist. Some varieties of land- 

 plants at present, as for example the mosses and liverworts, show 

 clearly in their structure their relationship to the algae, and 

 serve as connecting links between the great primal flora of the 

 ocean and the modern flora of the land. The algae, then, are 

 the forms from which all other plants are supposed to have orig- 

 inated. The history of life upon the earth is one of constant 

 improvement, and as land appeared improved forms of algae 

 tenanted it, and may have given rise to all the myriad higher 

 species of forest and prairie as they are now exhibited over the 

 continents of the world. And finally there are very many excel- 

 lent reasons for regarding the continents themselves to have 

 arisen largely through the activities of living organisms a proc- 

 ess that may be observed continuing even in these days if one 

 should visit the coral-islands of the south seas, those enchanted 

 atolls of the Pacific. 



