292 THE AMERICAN NATURALIST [Vol. XLIV 



Prepinus Jeffrey, which must be regarded as a very 

 antique representative of the pine-like conifers. This 

 is particularly significant on account of the other an- 

 cestral features presented by the nut pines. In both 

 the hard and soft pines of the present epoch we find a 

 gradual transition from the thick-walled rays cells with 

 abundant piciform pitting of the ancestral type to thin- 

 walled cells with few large compound pits. These large 

 pits have been formed for the most part by the fusion 

 of smaller pits, although in some instances they appear 

 to have been derived by the simple enlargement of the 

 piciform type of pit. It is of interest to note that the 

 succiniferous pines, which gave rise to the Baltic amber 

 in the late Eocene or early Oligocene, already show the 

 transition from the piciform to the large type of pit. 1 

 Conwentz notes that there may be from one to four 

 pits in each cross-field and that the number decreases 

 with the increase in size. He further notes that in 

 certain regions the pits are predominantly small and in 

 others large, but draws no conclusions as to the origin 

 of the latter. From a consideration of the mode of 

 formation of the large ray pits of Pinus, we see that 

 the two main divisions of the genus fall roughly into 

 two principal subgroups, one less specialized with one 

 or several small pits in each cross-field of the wood ray 

 and the other more highly specialized with large infre- 

 quent pits in thin- walled ray cells. These subgroups 

 grade into one another, but by recourse to other fea- 

 tures of anatomical specialization as well as to external 

 characters, a satisfactory basis for further subdivision 

 along true evolutionary lines is obtained. 



SUMMABY AND CONCLUSIONS 



1. By a study of the anatomy of fossil and living 

 pines certain lines of descent may be somewhat clearly 

 discerned. 



2. Cretaceous pines as well as Prepinus Jeffrey were 



i Conwentz, Monog. d. Bait. Bernsteinbaeume, pp. 55-56, Tab. XIV, 7, 

 Tab. X, 4. 



