16 EVOLUTION OF PLANTS 



by external influences, and as they represent presuma- 

 bly primitive conditions, the importance of a -study of 

 these early stages of the plant's existence is evident. 



When we consider the manifold sources of error, it 

 is not to be wondered at that botanists have not yet 

 been able to establish a perfect system of classification, 

 and that it must be a long time before anything ap- 

 proaching this can be hoped for. 



The plant kingdom is usually divided into a num- 

 ber of primary divisions, " branches," or "sub-king- 

 doms," as to whose limits there is not complete accord 

 among botanists. Excluding a number of groups of 

 doubtful affinity, sometimes put together under the 

 name Protophyta, botanists usually recognize the fol- 

 lowing sub-kingdoms : 1. Algae (green plants below 

 the Mosses); 2. Fungi (a group parallel with the 

 Algie, but destitute of chlorophyll); 3. Archegoniatae 

 (Mosses and Ferns); 4. Spermatophyta (seed-plants 

 the "flowering plants' of the older botanists). Of 

 these divisions, the Fungi and Algie are often united 

 into a single great division, Thallophyta, and the Arche- 

 goniatae divided into two sub-kingdoms, Mosses (Bry- 

 ophyta) and Ferns (Pteridophyta) ; but the arrangement 

 here given seems to the writer more in accordance with 

 what we know of the relationships of the different 

 members of these groups. 



