PINE TREES OF ROCKY MOUNTAIN" REGION. 19 



years after it was first described as a distinct species {P. monophylla 

 Torrey), it was reduced to the rank of a variety (P. edulis var. 

 monophylla Torrey) ; and 47 years later (1907) it, too, was desig- 

 nated as P. cembroides var. monophylla Voss, 1 a judgment in which 

 at least one author has concurred as late as 1909. 



The. present writer prefers, however, to consider the pihon and 

 single-leaf, pine as distinct species. For this opinion there is good 

 ground in the microscopic structure and external form of the leaves 

 of both trees, as well as in the size of their cones. Perplexing and 

 anomalous leaf forms of both trees occur, particularly where the ranges 

 of the two trees come together. Thus it is possible to find individual 

 trees in southern Utah bearing one-leafed and two-leafed fascicles, 

 but these must be considered P. edulis, because the apparently mono- 

 phyllous fascicles can usually be shown to be structurally two-leafed. 

 So also anomalous two-leafed fascicles of trees bearing mostly single 

 leaves will generally have the essential anatomical structure of P. 

 monophylla, to which such exceptional trees must be referred. 2 It 

 should be noted in this connection that similar variations from the 

 usual number of leaves borne are peculiar also to other western and 

 eastern pines. In the case of these species, however, such variations 

 have not yet been considered sufficient ground for uniting them as 

 varieties. 



DISTINGUISHING CHARACTERISTICS. 



The pihon has a short, often crooked, trunk, which gives off several 

 large, crooked branches and' is rarely clear of limbs for more than 

 15 feet, and usually for only 6 or 8 feet. Young trees have broad 

 cone-shaped crowns, but when the trees have become full grown 

 the crown is spherical or occasionally somewhat flat-topped. The 

 height attained varies from 10 to 35 feet or more, but commonly it is 

 from 12 to 16 feet, with a diameter of from 12 to 30 inches. The 

 irregularly developed, leaning, crooked trunks and low-hanging 

 branches of this pine give it the general appearance of an old apple 

 tree. • The bark of mature trunks is shallowly and irregularly fur- 

 rowed, the main ridges being joined by diagonally disposed smaller 

 ridges. Both the main ridges and the smaller ones are broken into 

 small, close, detachable scales. Superficially the bark has a tint of 

 yellowish or reddish brown. It varies in thickness from about one- 

 half to seven-eighths of an inch. 



1 See footnote 4, page 16. 



2 How far we have in the anomalous forms of these two pifions evidences of the deriva- 

 tion of a one-leafed species from the probably more ancient two-leafed tree it is impossible 

 to say at present. The somewhat more arid habitat, in part, of the one-leafed tree would 

 seem to support belief that the one-leaf form of foliage is the direct result of a psysio- 

 logical necessity — a leaf of such simple character as would permit the tree more easily to 

 maintain itself under arid conditions. 



