No. 521] THE EVOLUTION OF PINUS 



293 



characterized by thick-walled ray parenchyma and piri- 

 form lateral ray pits, by the absence of marginal ray 

 tracheids and by abundant tangential pitting of the 

 autumnal tracheids. 



3. The development of ray-tracheids, the disappear- 

 ance of thick-walled ray cells, the origin of large com- 

 pound ray pits and the loss of tangential pitting in the 

 autumnal tracheids are all features of the evolutionary 

 development of the pines of the present epoch. 



4. The large lateral pits of the rays in modern species 

 of Pinus have taken their origin for the most part by 

 the fusion of small pits with distinct circular borders. 



5. The hard and soft pines with very large lateral 

 ray pits are the most highly developed living pines. 

 The type of hard pines represented by P. resinosa in 

 North America and P. silvestris in Europe, represent 

 in the concurrent development of very large generally 

 solitary lateral ray pits, with dentate marginal ray 

 tracheids and the obliteration of tangential pitting in 

 the autumnal tracheids, except in the seedling and in 

 the woody axis of the- cone, the most highly developed 



6. The nut pines of North America 1 and Asia have 

 piciform lateral ray pits and thick-walled ray cells and 

 in these features approach most nearly to the conditions 

 of structure found in Cretaceous pines. 



7. The hard pines of the United States, with the ex- 

 ception of P. resinosa, show a great range of variation 

 from piciform to compound lateral ray pits. The soft 

 pines present a parallel series of gradation in ray 

 pitting. 



In conclusion, I wish to express my thanks to Pro- 

 fessor Jeffrey for material of Cretaceous and other 



