CHAPTER XIII 



THE PLANT KINGDOM 



Through a period reaching back into the past for milKons of 

 years, such a long time that the entire span covered by human 

 history seems almost negligible beside it, the evolutionary 

 advance of the plant kingdom has gone slowly but steadily 

 onward. We may still be uncertain as to the causes which lie 

 behind this tremendous progressive movement, but its results 

 are manifest in the scores of thousands of plant species with 

 which the earth is covered today. Those of us who are familiar 

 with the vegetation of the temperate zone, thriving and vigorous 

 though this may be, can have little idea of the luxuriance and 

 variety of plant life exhibited in tropical and subtropical regions. 

 In New England there are about 4,000 species of seed plants, 

 but probably 50,000 occur in Brazil, and in the whole world there 

 have already been described a vast array of almost 250,000 species. 

 Nor is our knowledge by any means complete. Although for the 

 past three hundred years botanical exploration has been active in 

 all parts of the globe, the discovery ofnew species is still constantly 

 being reported. It is the seed plants which constitute the 

 dominant and conspicuous part of this multitudinous vegetation, 

 and in the present volume we have confined our attention almost 

 exclusively to them, but we should remember that the plant king- 

 dom includes a host of lower and simpler members. Of the ferns 

 and their allies we now know more than 4,500 species, and in many 

 parts of the earth they are an important element in the plant 

 population. Of the liverworts and mosses there are some 16,000 

 species, and these are far exceeded by still lower forms, the fungi 

 with 60,000 species and the algae with 20,000. In all these groups, 

 exploration and critical study are yearly adding many new forms 

 and it is safe to estimate that, were our knowledge complete, we 

 should be able to recognize not less than 300,000 species of 

 plants. The day has passed when any one botanist can hope to 

 become familiar with more than a small portion of the flora of 

 the globe. 



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