No. 521] 



THE EVOLUTION OF PINUS 



285 



less range of variability and greater constancy in defi- 

 nite lines. On account of the long geological range of 

 Pinus, anatomical characters, notoriously more constant 

 than any others found in the higher plants, have had 

 time to become considerably diversified. As a result of 

 the action of this principle, it is much more easy to dif- 

 ferentiate two such species as Pinus strobus Linn, and 

 P. palustris Mill, from one another anatomically than 

 distinct Cupressineous genera such as CJunna-cyparis 

 and Thuya. This arises from the fact that the Cupres- 

 sineaB, to which the two last-named genera belong, are 

 of very recent origin compared with many of the species 

 of Pinus. 



Before attacking the anatomical differences which 

 mark the main lines of evolution in pines, it will be well 

 to consider the artificiality of the groups into which they 

 may be divided by the use of solely superficial characters. 

 Such characters are the number of leaves in the fascicle, 

 the deciduous or nondeciduous nature of the leaf-sheath, 

 the texture of the cone-scale and the relative size of the 

 seeds. Quinate leaf fascicles formed, for example, a 

 very satisfactory basis for the classification of pines 

 before those of the southern United States and Mexico 

 became known to science. In Europe, Asia and northern 

 North America, the possession of quinate leaf fascicles 



America, however, are found hard pines such as P. 

 torreyana Parry. P. arizonica Englin, and P. Monte- 



zuma Lamb, with leaf fascicle containing five leaves. 

 On the other hand, a persistent leaf sheath such as is 

 a feature of the hard pines is absent in P. chihuahuana, 

 P. leiophylla and P. Lumholtzii, quite typical pines of 

 the group in other respects. Nor is the texture of the 

 cone-scale or the position of the apophysis on the umbo, 

 characters which are used to distinguish the larger 



