I»2 



GYMXOSPERMS 



and increase the proportion of wood. The later fossil members and 

 all the living members have a small pith, large zone of wood, and a 

 comparatively small cortex. 



Whether these resemblances in stems mean genetic relationship 

 is conjectural. Ilelianthus, Sambuciis, and Cuciirhita have a large 

 I)ith, scanty zone of wood, and small cortex; but that does not mean 



that any one of them inherited 

 the condition from another or 

 that any of them got it from 

 anygymnosperm. Phylogenet- 

 ic anatomy is one of the best 

 indicators of genetic relation- 

 ships; but it is not always easy 

 to distinguish between phylo- 

 genetic anatomy and ecologi- 

 cal anatomy. Environment is 

 a powerful factor. If there 

 had been no changes in this 

 factor, the Cordaitales should 

 still be with us, and their 

 life-stories would be as well 

 known as that of Pinus. 



While the leaf is peculiarly 

 sensitive to changes in condi- 

 tions, and may change from 

 simple to compound in the 

 lifetime of an individual, we 

 should regard the compound 

 leaf as an outstanding char- 

 acter of the Cycadophytes; and the simple leaf as just as strong 

 a characteristic of the Coniferophytes. 



The two phyla existed side by side in the Paleozoic, and the 

 Cordaitales have been recognized much farther back than the Cy- 

 cadofilicales; but it is more than doubtful whether the Cordaitales 

 were the ancestors of the Cycadoiilicalcs. Both must have come 

 from heterosporous Pteridophytes. There was so much variety 

 among the Paleozoic Pteridophytes that it is not necessary to make 



Fig. 196. — Cordaiaulhus Grand d'Euryi: 

 section of beak of nucellus; wedged in the 

 passageway are two large pollen grains, the 

 lower one showing part of the surface of the 

 exine and also the multicellular interior; 

 X225. — After Renault. ■'''5 



