614 PROCEEDINGS OF THE AMERICAN ACADEMY. 



These facts indicate that the Abietineae are a much older group geo- 

 logically than is usually supposed. It is further evident that such 

 forms as these must be the ancestors of living pines, and that such 

 forms as Pinus scituatensiformis and P. succinifera, — • of the same or 

 later geological age, yet less specialized, — are off the main line of 

 development. 



Before leaving this specimen, it is convenient to consider the light 

 it throws on the origin of ray tracheides. Jeffrey and Chrysler (11) 

 concluded that ray tracheides were evolved during the early Tertiary, 

 basing their conclusions on the following developmental and palaeobo- 

 tanical evidence. Ray tracheides are absent from the cone axis of 

 most modern pines, and poorly developed in the seedling; they are 

 absent in Pinites Ruffordi, Seward (12) (Wealden), Pityoxylon staten- 

 sense and P. scituatense (Middle Cretaceous) and do not appear for 

 several years' growth in Pinus succinifera (Early Oligocene or Late 

 Eocene). The discovery of ray tracheides in P. scituatensiformis 

 (Middle Cretaceous) led Mr. Bailey to the conclusion that these 

 structures came in during the Middle Cretaceous. In that species 

 they do not appear at all in the first ten to fifteen years' growth and 

 thereafter are but poorly developed. Their occurrence, though rare, 

 in the first annual ring of P. protoscleropitys (Middle Cretaceous) 

 and their abundance later, seems to indicate that they are a more 

 ancient feature than has been assumed by any of the above cited 

 investigators. It is probable that they were developed in the Lower 

 Cretaceous if not in the Jurassic. 



As regards the origin of ray tracheides, the final word remains to 

 be said. There are two theories which have been advanced to explain 

 the question. Thompson (13) has suggested that tracheary ray cells 

 are derived from vertical tracheides, which by progressive shortening, 

 have taken on a horizontal position. Stages in such a process he found 

 in Pinus resinosa and P. strobus. As Bailey pointed out there are two 

 objections to this theory, — if these phenomena are recapitualtionary 

 or reversionary, in the first place, why are they more evident in these 

 highly specialized varieties than in such primitive ones as the Nut 

 and Foxtail pines? And, in the second, why are they completely 

 absent in fossil forms? Mr. Bailey was unable to find any trace of 

 such an origin in P. scituatensiformis, and I have been unable to find 

 any in P. protosclerojntys. Since there is no confirmatory evidence in 

 the case of the primitive living forms, or the two oldest known fossil 

 forms, it seems improbable that Mr. Thompson's interpretation is 

 correct. 



