HISTORY OF PLANTS. 1/3 



already pointed out, the similarity in structure would lead us to be- 

 lieve that the Silurian land plants were simply terrestrial forms of 

 the marine algae. The same conclusion would follow from the study 

 of their anatomy and development. The rhizocarps, to which some 

 of the early land plants seem to be related, are themselves closely 

 related to various algae. So, too, may the ferns and horse tails 

 or the Carboniferous be regarded as derived from lower marine 

 plants (algce), with various modifications, probably through some 

 lost intermediate steps. The cone bearers, however, give evidence 

 of having descended not from the algse, but from some high form 

 of plant belonging to the Lycopoda. Their structure, and espe- 

 cially their method of reproduction and development, shows them 

 closely related to certain plants classed with the Lycopoda (Isoetes), 

 but differing from the ordinary Lycopoda in having two kinds of 

 spores, a large one and a small one. These micro- and macrospores 

 correspond to the pollen and embryo sac of the flowering plants, and 

 from some such source doubtless have the latter been produced. 

 The endogens again were unquestionably not products of the cone 

 bearers, but they had an independent origin down the main line, not 

 unlikely an origin close to that of the gymnosperms. The higher 

 flowering plants, the exogens, though closely related to the gym- 

 nosperms, were probably not derived from them directly, and were 

 certainly not derived from the endogens. Probably they had an 

 independent origin from some lower group close to the gymnosperms. 

 It would seem then that some low algae-like type of plant at one time 

 became terrestrial, and then there occurred a divergence from it 

 which resulted in the various forms of cryptogams, the mosses, ferns, 

 and lycopods. These groups soon expanded and reached their cul- 

 mination in the Carboniferous. But along one line of descent 

 (Isoetes, etc.,) two kinds of spores were produced, and this line now 

 expanded and produced the flowering plants, the gymnosperms, 

 endogens, exogens, all of which plants retain the t\vo kinds of 

 spores (pollen and embryo sac), but seem to give indication of having 

 had independent origins from the cryptogams. 



It is a fact of no little interest that the flora of the world probably 

 did not, as would be expected, originate in the tropical regions, but 

 on the contrary in the Arctic zone. Abundant evidence shows that 

 in the northern regions the various groups of plants first appeared 

 and culminated. This must, of course, indicate that the climate of 

 the Arctic zone was not the frigid one that it is to-day. 



