THE GYMNOSPERMAE : CYCADALES, ETC. 



731 



the meristem organizes only leaf traces and the typical vascular cylinder 

 is built up by their convergence in the central region. The first four sets of 

 leaf traces are directly connected to the vascular plate, but the subsequent 

 sets of traces arise from the preceding traces. This is the familiar pattern 

 of the Dicotyledon stem where, however, the protostelic seedling plate has 

 disappeared and leaf and root strands are directly linked to one another. 

 Here, in this seedling plate, seems to lie the last relic of the extensive cauline 

 stele of the Vascular Cryptogams, and above the Cycad level in the story 

 of plant evolution we are faced with a new method of stem organization from 



C^cas Plant 



Male Cone 



Female Sporophyll 

 Microsporangiu 



Oospore 





fAegasporangium 



Male Nuclei 



Embryo Sac 



~^ Antheridiat Cell 

 Fig. 734. — Life-cycle of Cyras revoluta. 



leaf traces or " vascular bundles " which cannot be directly compared with 

 the stelar organization of the lower plants. The flexibility of organization 

 possible with the new method has evidently been a great gain, and it is not too 

 much to say that the successful development of the Seed Plants has largely 

 depended upon it. The fossil evidence shows that grafting the seed habit 

 upon a cryptogamic vascular system did not succeed. 



Ginkgoales 



There is only one living type included in this order, the species Ginkgo 

 hiloba. The characters of the order are therefore those of the species. Like 

 Cycas it retains the cryptogamic character of fertilization by antherozoids. 



This 



Ginkgo hiloba (The Maidenhair Tree) 

 species has a good claim to be regarded as the senior living 



plant. Its peculiar leaves are found with certainty in the Permian, and with 



