12 FLORA OF THE YELLOWSTONE NATIONAL PARK. 



Black Pine in amount and distribution. It is found throughout 

 in cool, moist situations, at low elevations on the northern 

 slopes, and especially common on wet subalpine slopes and 

 plateaus about the timber line, forming groves in the park-like 

 openings. Moist plateaus above 8000 feet and the slopes and 

 bottoms of deep canons are forests of this species and Picea 

 Engelmanni, these two trees forming at least 25 per cent, of 

 the forest area of the Park. The Spruce (Picea Engelmanni, 

 Engelm.) is associated with the Balsam. It is not of great 

 size, rarely more than 2 or 3 feet in diameter, and does not form 

 extensive forests as in the central Rocky Mountain region, and 

 still farther northward in the mountain region of Montana 

 becomes rare and of small size. 



Picea alba, Link, which occurs in the Black Hills of Dakota 

 and in northern Montana, reaching its greatest development in 

 the Flathead Region, probably does not occur within the Park, 

 although some of the cones of Picea Engelmanni show a transi- 

 tion into P. alba. This fact is suggestive, occurring, as it does, 

 in a region between that of the greatest development of P. 

 Engelmanni on the south, and P. alba on the north ; although 

 in northwest Montana, where both species occur, Prof. Sargent 

 has observed the same fact, but they are found " at different 

 elevations in different soils and never mingle." 



Picea pungens, Engelm., doubtless does not enter into the 

 Park forests. Specimens collected under the name of Abies 

 Menziesii, Lindl., are probably forms of Picea Engelmanni. 



The Red Cedar (Juniperus Virginiana, L.) grows along the 

 Gardiner River, a few miles above the mouth and mingled with 

 Pinus flexilis over the formation of the Mammoth Hot Springs. 

 It is rare elsewhere. 



Juniperus communis, L., var. alpina, Gaud, occurs sparingly 

 on rocky slopes at low elevations and more frequently about the 

 hot spring areas. On the moist slopes and along the streams 

 of the lower grass-covered areas are frequently groves of Popu- 

 lus tremuloides, Michx. (Aspen). It was observed sparingly 

 above 8000 feet. Populus augustifolia, James, was only seen on 

 Cache Creek. 



Of the shrubs that are common throughout may be mentioned, 

 Betula glandulosa, Michx. ; Salix desertorum, Rich., var. Wolfii, 



