1 68 THE LIVING WORLD. 



and their lack of any complicated system of organs 

 makes their embryological history less significant 

 than that of animals. We can, it is true, determine 

 that all plants start their history as single cells, and 

 this would seem to indicate that, like animals, they 

 have been originally derived from unicellular ances- 

 tors. But it is impossible to find traces of anything 

 in the history of plants that correspond to the 

 Gastraea of the animal world, nothing that can be 

 regarded as the central starting-point from which the 

 various groups diverged. Indeed, the history of 

 plants seems more like the history of a single de- 

 veloping line of life than of a series of diverging 

 lines. 



Although embryology gives us little help in study- 

 ing the history of plants, still by combining its 

 teachings with the facts derived from comparative 

 anatomy, some few points can be determined in re- 

 gard to the line of descent through which plants 

 have come. That the unicellular plants were fol- 

 lowed by the low algae cannot be questioned. That 

 the higher plants were derived from these seems 

 also sure. The mosses, ferns, club mosses, cycads, 

 cone bearers (pines), and angiosperms (common 

 flowering plants) follow each other in something like 

 the order given, with increasing grades of structure. 

 Now in the embryology of the cone bearers (gym- 

 nosperms) we can still find traces of an earlier stage 

 in the history of plants corresponding to the club 

 mosses, and even in the highest plants of all, the 

 angiosperms, there are indications of a like history. 

 It is significant to find that the fossil history of 



