304 MORPHOLOGY OF GYMNOSPERMS 



from the Triassic of Sweden, and concluded that pines flourished at 

 that time. These discoveries carry the Abietineae, represented by 

 Pinus and allied forms, through the Mesozoic, in a position to be 

 connected with paleozoic gymnosperms. It must be recognized that 

 cones and even winged pollen grains may not be sure evidence of 

 Abietineae during the early Mesozoic, but they must be regarded as 

 at least probable evidence. 



The more detailed connection of living Abietineae with the paleo- 

 zoic Cordaitales has been made by Jeffrey (142) in his investigations 

 of the gymnosperm remains from the Cretaceous of Staten Island 

 (N.Y.), partly in connection with Chrysler (112) and Hollick 

 (155). The investigations involved the development of a special 

 technic in the treatment of fragments obtained from clay pits. The 

 wood of these pines of the Middle Cretaceous, known as Pityoxylon, 

 differs from that of living pines in the absence of marginal ray tra- 

 cheids, highly resinous rays, and an association of certain features of 

 hard and soft pines. It is inferred that these characters are ancestral 

 for Pinus since they persist in the cones of existing species (112). 

 It was in connection with an investigation of the structure of the 

 leaves of cretaceous pines, however, that the most interesting 

 connection was made, for it resulted in the recognition of what is 

 regarded as the direct ancestor of Pinus, which received the generic 

 name Prepinus (142). This interesting form had deciduous dwarf 

 shoots bearing numerous spirally arranged leaves, whose structure 

 is most suggestive. There are paired resin canals continuous to the 

 very base, well-marked centripetal xylem, and about the vascular 

 bundles a complicated double sheath of transfusion tissue resembling 

 that found in the leaves of some Cordaitales. Many of the true pines 

 of the Cretaceous were found to have this sheath, but not the centripe- 

 tal xylem; and Jeffrey regards the tissue adjacent to the protoxylem 

 in the leaves of existing pines as a relic of the inner transfusion 

 sheath rather than of centripetal xylem (p. 10). In this way a con- 

 nection is established between living pines and paleozoic Cordaitales 

 through cretaceous pines and Prepinus. If this connection is trust- 

 worthy, the Abietineae are as old as any tribe of the Pinaceae can be, 

 and Pinus is the oldest genus. 



If Sequoia be excluded from the Taxodineae, there is no evidence 



