No. 521] 



THE EVOLUTION OF PINUS 



287 



has throughout the single leaf bundle, the single row of 

 resin canals in the first woody ring and smooth-walled 

 marginal ray-tracheids. 1 These two great races of pines, 

 which may be traced from the Cretaceous to the present, 

 may be subdivided on the basis of other characters, in- 

 ternal and external. "Within the subdivisions differ- 

 ences in floral and foliar characters are much less vari- 

 able than in the main lines. 



The nature of the parenchyma in the wood rays and 

 the characters of the pits which connect them with the 

 tracheids are features of considerable evolutionary and 

 diagnostic importance in the various smaller subdivi- 

 sions of the genus under consideration. For example, 

 among the soft pines we find a group illustrated by the 

 nut pines and foxtail pines of the United States, and 

 by the nut pines of Asia (P. Gerardiana Wallich. and 

 P. Bungeana Zucc), which are characterized by the pos- 

 session of thick-walled ray parenchyma, with very small 

 lateral pits having distinctly circular borders (Fig. 1). 

 These pits are usually four in each 1 ' cross field" (a 

 cross field is the area of intersection of a ray cell and a 

 tracheid) and quite distinctly separated from each other. 

 The opening on the side of the tracheid is slit-like and 

 often exceeds in length the diameter of the pit. The 

 opening on the side of the ray cell is circular or lentic- 

 ular and smaller or nearly equal in diameter to the 

 border. In most of the nut pines and fox-tail pines 

 these pits are rather uniform in size and in diameter, 

 the same as the lateral pit in the marginal tracheids of 

 the rays. In P. Bungeana of China the pits are of 

 large size, with widely lenticular openings and more 

 approximated than in the other nut pines. Small lateral 

 pits such as those just described with distinct circular 

 borders are characteristic of the ray cells of most of 

 the conifers and are conveniently termed "piriform." 



