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, a judgment in which 
| at least one author has concurred as late as 1909. 
The present writer prefers, however, to consider the pifon 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.” 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 pifion 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 
_Tidges. 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. : 
1See footnote 4, page 16. 
2 How far we have in the anomalous forms of these two pions 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 hecessity—a leaf of such simple character as would permit the tree more easily to 
Maintain itself under arid conditions, 
