no Plant Life and Evolution 



have been made within recent years. It is now evi- 

 dent that many fossil plants, formerly supposed to 

 be true ferns, are really seed-bearing plants of a 

 very primitive kind; and it is practically certain 

 that these primitive seed plants have originated more 

 than once from quite distinct types of ferns. It is 

 clearly proved that two of the lowest types of ex- 

 isting seed plants, the Cycads and the curious 

 Ginkgo, or maiden-hair tree, are related to the ferns. 

 Some of the fossil club-mosses, e.g.; Lepidocarpon, 

 were also seed-bearing plants, and there are some 

 pretty strong reasons to believe that these, or forms 

 not unlike them, may have been the progenitors of 

 another characteristic group of existing seed-plants, 

 the Conifers. 



While a few of the Paleozoic horsetails were cer- 

 tainly heterosporous, heterospory was less perfectly 

 developed in this group, and as yet there is no evi- 

 dence of any seed-bearing plants that can be con- 

 sidered to be allied to the horsetails, and it is proba- 

 ble that this phylum never advanced beyond the 

 heterosporous phase. 



The Nature of the Seed. The seed is a further 

 elaboration of the megasporangium. In the seed- 

 plants the latter generally remains attached to the 

 sporophyte until the spore, which is permanently re- 

 tained within, has completed its development and 

 fertilization has been effected. The embryo-plant, 

 enveloped in the double covering of the spore mem- 

 brane and the wall of the sporangium, is very ef- 



