382 



SCIENCE. 



[N. S. Vol. VIII. No. 195. 



as a conifer. Long before the evidence of 

 spermatozoids was discovered it seemed per- 

 fectly clear to me that Ginkgo was more 

 cycad-like than conifer-like. In the light 

 of our present knowledge the appearance of 

 Ginkgo in association with the Carbonifer- 

 ous cycads seems natural enough. It is 

 a matter of very secondary importance 

 whether we are to regard it as an inde- 

 pendent line or not. I am inclined to be- 

 lieve that, while during the Palteozoic hete- 

 rospory and the seed were both attained, 

 siphonogamy was in its beginnings, and that 

 the spermatozoid habit was for the most 

 part still continued in the seed line. There 

 is no conclusive evidence, therefore, that 

 any of our modern coniferous genera ap- 

 peared during the Palaeozoic, during which 

 the Cordaites were the dominating seed 

 plants. During the last Palaeozoic periods 

 undoubted conifers did appear, and in con- 

 siderable abundance, and we may recognize 

 the beginnings of distinct lines represented 

 to-day by Abies and its allies, Taxodium 

 and its allies, and Taxus and its allies, but 

 the genera are not those of to-day. In the 

 lower Mesozoic, however, modern arauea- 

 rian and abietinous genera appear ; and the 

 Taxodium and Taxus lines become more 

 distinct, but not modern until the later 

 Mesozoic. At that time Cupressus forms 

 also appear, but not of modern genera. Fur- 

 ther details are not necessary, as the point 

 to be made is that the conifer type was not 

 recognizable until late in the Palaeozoic, 

 and then not in its modern expression. It 

 certainly suggests a later departure from 

 the Cordaites stock than do the cycads. 



Another fact is interesting to note in 

 connection with the evolution of the conifer 

 forms. In existing conifers there is con- 

 siderable variation in the development of 

 the male gametophyte. In some forms, as 

 the Abietineae, the development of two or 

 three prothallial cells, distinct from the 

 large antheridial cell, is a well-known fact, 



an amount of prothallial development not 

 shown by any other living heterosporous 

 forms, even the heterosporous pteridophj^tes. 

 In other forms, as Cupressineae and Taxeae, 

 the reduction of the male gametophyte is 

 greater, no sterile prothallial cells appear- 

 ing, the whole structure being an anthe- 

 ridium, as in the angiosperms. Our histor- 

 ical evidence accords with this progressive 

 reduction of the male gametophyte, the 

 Taxus and Cupressus lines having attained 

 modern expression after the Abies line ; 

 and back of the Abies line we find the Cor- 

 daites, with probably a still greater de- 

 velopment of the sterile region of the 

 male gametophj^te indicated. To derive the 

 Cordaites or Abies lines, with their two or 

 three to many-celled sterile tissue of the 

 male gametophyte, from such heterosporous 

 lycopod forms as we know to-day, with 

 their constantly more reduced male gamet- 

 ophytes, is not within the bounds of prob- 

 ability. Besides, the reduction of the male 

 gametophyte seems to be so prompt a re- 

 sponse to heterospory that its partially 

 reduced condition in certain conifers, and 

 probably in Cordaites, would seem to argue 

 for their near derivation from some homos- 

 porous type. 



The development of a suspensor in the 

 lycopod forms has also suggested a genetic 

 connection with gymnosperms, in which 

 the suspensor development is so conspicu- 

 ous. This organ, however, seems to have 

 no morphological constancy. In gymno- 

 sperms it may be developed from a plate of 

 cells formed in the oospore, as in most coni- 

 fers ; or from a mass of cells formed basally 

 or parietally in the oospore, as in cycads ; 

 or from free cells formed within the oospore, 

 as in Ephedra ; or from the elongation of the 

 oospore itself, as in Gnetum ; or from the 

 downward elongation of the archegonium, 

 as in Welwitschia. The suspensor, there- 

 fore, seems to be a temporary organ of the 

 embryo, of various morphological origin, 



