The range of Douglas fir covers most of the Rocky Mountain region in the United States and northward to central British Columbia; on the coast from the latitude of southern Alaska to the Sierra Nevada mountains in central California. It reaches its maximum development in western Washington and Oregon, particularly between the Cascade mountains and the Pacific ocean. In these Cascade forests, stands are found which yield from 50,000 to 100,000 feet per acre, and mills in that region cut the longest timbers in the world, some two feet square and 100 feet long. Two forms of Douglas fir are recognized by botanists, not essentially different except in size and habit of growth. One is the finely developed form on the Pacific coast where the climate is warm and the air moist. The other is the Rocky Mountain form which is smaller and shows the effect of cold, dryness, and other adverse circumstances. When the seeds of the two forms are planted in nurseries, where they enjoy identical advantages, the coast form outgrows the other in Europe, but the Rocky Mountain form thrives best in the eastern part of the United States. Douglas fir needles are from three-quarters to one and a quarter inches long, and of a dark, yellow-green color. They remain on the twigs about eight years. Cones are from two to four and a half inches long, and are borne on long stems. The seeds, which ripen in August, are of light, reddish-brown color with irregular white spots on the lower side; are about a quarter of an inch long, and are provided with wings. Trees of this species in the moist climate of the Pacific slope average much larger than those in the mountains farther east. The largest are 300 feet high, occasionally more, and from eight to ten in diameter. The average among the Rocky Mountains is from eighty to 100 feet high, and two to four in diameter. Young trees are slender with crowded branches. In thick stands the lower limbs die and the trunks remain bare, except an occasional small branch. Douglas fir at its best grows in thick stands, with crowns forming a canopy so dense that sunlight can scarcely reach the ground. The result of this is that other species have little show where Douglas fir prevails. The bark of large trunks attains a thickness of eight or ten inches near the base. Young bark contains blisters filled with resin, similar to those of balsam and other species of fir.