CORDAITALES AND GINKGOALES 315 



opening mechanism of the microsporangia. The microsporophyll 

 of Ginkgo accordingly has a double interest from the evolutionary 

 standpoint, for it not only shows the centripetal or cryptogamic 

 wood more clearly than it is exhibited by the vegetative leaves, 

 but at the same time manifests its transition by imperceptible 

 gradations into the mechanical tissues of the sporangium wall. 

 As has been indicated in an earlier chapter, Ginkgo is the lowest 

 type in which the dehiscence of the microsporangium no longer 

 depends on an annulus derived from the epidermis but is effected 

 by an internal mechanism derived from the old centripetal or 

 cryptogamic wood of the fibrovascular bundle. 



In the stalk of the ovule centripetal elements and transfusion 

 tissue are also well developed, but they apparently do not at any 

 time penetrate into the substance of the megasporangium. It is 

 not unlikely that tracheary tissues of a transfusionary nature were 

 formerly present in the megasporangial structures of the Gink- 

 goales, but that in the course of time they have suffered abortion. 

 The organization of a number of seeds of Paleozoic age of unascer- 

 tained affinities is good evidence in favor of the probability of this 

 view. Moreover, in one of these, Stephanos permum, characterized 

 by a tracheary mantle in the wall of the nucellus ending in the 

 pollen chamber, pollen grains are present, winged all around and 

 strongly resembling those of Ginkgo. It is accordingly not im- 

 possible that Stephanos permum was the seed of some Paleozoic 

 representative of the Ginkgoales. 



The importance of the sole surviving and monotypic genus 

 Ginkgo from the standpoint of the evolutionary transition from 

 the ancient to the modern gymnosperms cannot be overestimated. 

 It constitutes virtually a link between the Archigymnospermae 

 and the Metagymnospermae, since it presents to so large a degree 

 the characteristics of both. Its affinities on the lower side are 

 clearly with the Cordaitales, as has been recognized by all com- 

 petent investigators in recent years. Its relationship with the 

 Abietineae among the Coniferales is equally well indicated by 

 comparative anatomical and paleobotanical data, as will be shown 

 in the following chapter. 



The indications of relationship with the Cordaitales are pre- 

 sented in connection with the organization of the wood in primitive 



