46 BULLETIN 460, U. S. DEPARTMENT OF AGRICULTURE. 
Moreover, the habit this tree has of long retaining its cones ina ~ 
closed state enables the species to perpetuate itself even after severe 
fires have killed the mother trees, because, if not too badly burned, 
many of the charred cones then open and liberate their seed. Vast 
areas in the central and eastern parts of its range once heavily 
stocked with Pinus strobus and P. resinosa are now re-covered with 
jack pine, which is often the only growth on these barrens. 
LONGEVITY. 
Jack pine is comparatively short-lived, the largest trees probably 
not exceeding from 125 to 150 years of age. Asa rule, however, the 
age of such trees can not be determined accurately, because they are 
decayed at the heart. The oldest sound trees ordinarily cut rarely — 
exceed 110 years in age. Trees from 6 to 15 inches in diameter grown ~ 
in an open stand are from 25 to 85 years old, while 6-inch trees grown 
in a dense stand are from 70 to 75 years old. 
KEY TO SPECIES. 
Leaves borne singly (or some of them 2 in a bundle)_______ Pinus monophylla 
Leaves borne in bundles of 2: 
Cones armed with delicate prickles_______________________ Pinus contorta 
Cones without prickles— i 
Cone scales widely open after the cones are ripe__________ Pinus edulis 
Cone scales mostly closed. or only part of them open, after the 
CONES ‘ATE! TIPS Wessel CEE a ae ees eee Pinus banksiana 
Leaves borne in bundles chiefly of 3 (sometimes in bundles of 2, 4, or 5): 
With a prominent basal sheath— 
Cone scales armed with stout prickles— 
Leaves mostly from about 4 to 7 inches long______ Pinus ponderosa 
Leaves mostly from about 8 to 15 inches long____ Pinus apacheca 
Without a basal sheath: 
Cones borne on long, slender stems_______________ Pinus chihuahuana 
Cones borne on very short stems________._-_-_-_-_-_-_-_ Pinus cembroides 
Leaves borne in bundles of 5 and cones with prickles: 4 
Cone scales armed with long, delicate prickles______________ Pinus aristata 
Cone scales armed with short stout prickles________________ Pinus arizonica 
Leaves borne in bundles of 5 and cones without prickles: 
Cones opening at maturity and shedding their seeds— 
Cone scales strongly reflexed at the tips___________ Pinus strobiformis 
Cone scales mostly not reflexed at their tips— 
Seeds with long, well-defined wings______________ Pinus monticola 
Seeds with very short rudimentary wings__________ Pinus flezrilis 
Cones remaining closed at maturity______________________ Pinus albicaulis 
