THE ORIGIN OF THE VASCULAR PLANTS. 289 



the Angiosperms directly from some group of Pteridophytes. 

 The peculiar genus Isoctcs has been suggested as a form 

 possibly resembling this ancestral type, but it is still an 

 open question to which group of homosporous Pteridophytes 

 Isbetes is most nearly related. 



The essential points of structure of the Phanerogams, 

 the seeds and pollen tube, are only further developments 

 of heterospory, and may very readily be conceived to 

 have arisen independently more than once. The structure 

 of the Angiosperms is too constant to make it probable that 

 the Monocotyledons and Dicotyledons are not intimately 

 related, but among the Gymnosperms the diversities of 

 structure are so great that it is quite possible that the 

 Conifers and Cycads, for example, may not be at all re- 

 lated, but constitute two quite separate lines of development. 



RECAPITULATION. 

 To sum up then what we have endeavoured to state in 



the foregoing pages, it is generally admitted that the origin 



of the vascular plants is to be sought among the less 



specialised Bryophytes, which in turn are unquestionably 



derived from algal ancestors. There is, however, diversity 



of opinion as to whether the first vascular plants came from 



forms similar to those existing at present, or from forms 



immediately between them and the Algae. 



It is further admitted that in the evolution of the 

 sporophyte it gradually passed from a condition where its 

 whole substance was devoted to spore-formation to that 

 where more and more spore-formation was subordinated to 

 the vegetative life of the sporophyte. This now, by the 

 development of special organs, roots and leaves, became 

 entirely free from the gametophyte upon which in the lower 

 forms it had lived as a parasite. Of the living Bryophytes, 

 or non-vascular Archegoniatae, the genus Anthoceros comes 

 nearest to realising this condition. 



From a condition like that found in Anthoceros it is 

 quite probable that more than one line of development has 

 proceeded, resulting in the different groups of Pteridophytes, 

 which in their turn may have given rise to seed-bearing 

 plants quite independently of one another. 



Douglas Houghton Campbell. 



