No. 521] THE EVOLUTION OF PINUS 291 



these may be represented by several small pits, and 

 transitions may be found showing the development of 

 these large pits from the smaller ones by fusion. In 

 Fig. 5 is seen a condition characteristic of P. strobus 

 Linn., P. silvestris Linn., P. Laricio Poiret., P. Thun- 

 bergii Pari., P. Koraiensis Sieb. et Zucc, P. resinosa 

 Sol., P. Cembra Linn, and other pines with large lateral 

 ray pits. In the upper part of Fig. 5 the stages in the 

 development of two large pits from four small ones may 

 be made out, while in the central region appears the 

 origin of a single large pit from two smaller ones. In 

 the lower part of the same figure may be seen instances 

 of multiple fusion, such as are particularly character- 

 istic of the soft pines, where the pit openings on the 

 side of the tracheids often fuse before those on the side 

 of the ray, so that the ghost-like relic of the separating 

 ray wall may still be observed. In Fig. 4 is seen a con- 

 dition, which may be considered to illustrate the forma- 

 tion of pits of medium size in Fig. 5. One or two rather 

 small pits are being formed in most cases from the 

 fusion of from two to four very small piciform pits. 

 In Fig. 3 is seen a condition characteristic of certain 

 soft pines (P. Avacuite Ehrenberg, P. albicaulis Englm., 

 etc.) where the large-bordered pits have formed and are 

 fusing in places to form elongated pits with oval out- 

 lines. The large pitted soft and hard pines show in the 

 seedling a reversion to the ancestral condition of numer- 

 ous small pits and in their cone-axes piciform ray pits 

 are characteristically present. Such pines have ordi- 

 narily thin-walled rays cells, with the exception of such 

 intermediate species as P. Lambertiana Doug, and P. 

 Ayacuite Ehrenb. 



With this review of the ray pitting in modern pines, 

 it will be apparent that in the nut and fox-tail pines we 

 have exclusively piciform pitting in the lateral walls of 

 the ray. This condition is of interest because it per- 

 petuates the type of pitting which has been invariably 

 found in the rays of Cretaceous pines as well as in 



