1080 Nienburg: Morphologie der Gewebe (Anatomie) 1911. [4Q 



earliest and most primitive forms the tendency in the vascular System of the 

 stem and leaf for the protoxylem to become continuous and in seriation witb 

 the centripetal wood, and for an ultimately exarch condition to result. 



These two groups are also clearly separated on other evidence. The 

 endarch series are very femlike in habit, possess seeds of a peculiar type, 

 and show in their petiolar bundles (at least in Lyginodendron) that ancestral 

 mesarch condition of the Botryopterideae where the protoxylem shows no 

 preference for either the inner or the outer wood. The early members of the 

 exarch group, however, such as Medullosa do not show such striking res- 

 emblances to the ferns, possess seeds of a higher and more cycadean type, 

 and display an exarch condition not only in the leaf trace but troughout the 

 petiolar system. 



The double trace is clearly present in the endarch series, and is also 

 found in most of the exarch forms, iiicluding the higher CycadofiUces ; Por- 

 oxylon, Cordaites, and their allies; many modern conifers, and the living Cycci' 

 dales. It seems clearly to have been a primitive character, and to have arisen 

 from a constriction of the single diarch Strand which was present in the 

 •ancestors of all seed plants. 



The endarch line of development apparently ended blindly and did not 

 give rise to the typically endarch higher plants, which were developed along 

 several distinct lines from a plexus of forms in the exarch group pössessing 

 a double leaf trace and seeds resembling those of Medullosa. 



The Poroxyleae, Pityeae, and Cordaiteae camo from this plexus at a very 

 remote period. The tendency among this series has been toward the deve- 

 lopment of a parallel veined leaf, a clearly gymnospermous type of repro- 

 duction, and an endarch condition of the central cylinder, together with a 

 wide zone of secondary wood. 



The parallel venation of the leaf has doubtless been derived from a 

 pinnate condition, with a two-bundled exarch rachis, a structure which pas 

 persisted only at the base of the trace. The cordoitean type of leaf seems 

 to have existed in the Uretaceous, as is shown by the occurrence at that 

 period of such forms as Niponophyllum. The genus Nüssonia, pössessing a 

 simple leaf with a distinct midrib but with clearly parallel veins, shows an 

 intermediate condition between a pinnate cycadean leaf and a parallel veined 

 condition, and gives a Suggestion as to how the latter may have been 

 produced. 



The cordaitean type of i-eproduction is unknown in the CycadofiUces, 

 but Trigonocarpon, the seed of Medullosa, approaches' closely the seeds of the 

 Cordaiteae. 



The endarch condition of the central cylinder has been caused by the 

 disappearance of the centripetal primary wood of an exarch stem consequent 

 upon the great increase in bulk of the centrifugal secondary wood, to which 

 the protoxylem has finally attached itself. 



A second line from the double-bundled Medullosa-like plexus of the 

 exarch group, but one which lies close to the cordaitean alliance, has given 

 rise to the Ginkgoales, the Coniferales, and probably the angiosperms» 

 Centripetal wood persists here only in Ginkgo, and in the leaves of such 

 ancient conifers as Prepinus. Endarchy is apparently elsewhre universal, 

 although our knowledge of conditions in the earlier forms is very slight. 

 Ginkgo and the more primitive conifers possess a double foliar bündle, and 



