FOSSIL BOTANY. 



243 



not yet fully explored, and there is no means, therefore, of 

 knowing the number of species of plants which exist. 

 Even iu countries long known new species, especially of 

 the lower plants, are constantly being found. In 1824 the 

 total number of species was estimated to be over seventy 

 thousand; in 1841, over seventy-eight thousand; and in 

 1845, over ninety-two thousand; now the estimates are as 

 high as one hundred and fifty thousand to two hundred 

 thousand, or over. The aifinities of the different groups — 

 whose accuracy is at best, of course, only approximate — is 

 shown in the following diagram (taken, by permission, from 

 Bessey's Botany) : 



Gamopetalse. 



Apetalse. 



MoniKotyledones. 



ChoripetalsB. 



Dicotyledones, 



Angtosfierma, 



Cyntnosperma. 



Pteridopbyta. 



Bryophyta. 



Carfospokea, 



Oospores. 



Zygospores. 



ProiophytAi 



