344 BOTANICAL GAZETTE [November 



on the ridges and with the Populus angustifolia formation in the 

 canon bottoms. Pseudotsuga mucronata is more common than the 

 other characteristic of the formation and is far more extensive in 

 its habitat than the latter. Besides Picea Engelmanni^ P. Parryana 

 occurs in the formation, forming a straggHng society along the bottom 

 of the canons from 2100°^ to 2700™. Both of these species are 

 confined to the cool, moist canon bottoms and the draws running into 

 them. The Pseudotsugas, on the contrary, extend up on to the dry 

 ridges to associate with the pines growing there. They come down 

 the canons to 1710"^, while the lowest limit of Picea is about 1950°*, 



I have already mentioned the fact that Pseudotsuga, in the lower 

 part of its range, is restricted to the north slopes.^ ^ A comparison of 

 the physical conditions of the opposite slopes of Left Hand Canon at 

 an approximate altitude of 2100^ was made by me on Septernber 20, 

 1905. Two stations were selected, one on the south side (north 

 slope) of the caiion and the other on the north side (south slope). 

 The former slope was well covered with aspens, the station being 

 chosen in a little glade among them. The latter slope was sparsely 

 forested with aspens and pines (P. scopulorum). Each station was 

 about g"^ above the stream. The former I have designated as A 

 and the latter as B. The following are my results, the observations 

 being made September 20, 1905: 



r 



Temperatures 



Station A, 2:15 p. m 7?5 9?5 io?5 20^25 2i?7 



Station B, 2:50 p. M 22?5 19^ 43° 28^5 25?$ 



in 



Relative humidity 



y^^cm (jam 



Station A, 2:15 p. m:.. 32 per cent. 27 per cent. 



Station B, 2:50 p. m.. 25 per cent. 26 per cent. 



Water content * 



i°* depth 22 . s'^^ depth f 



Station A 4.9 per cent. 11 .3 per cent. 



Station B 2.7 per cent. 3 . 3 per cent, 



♦The preceding evening was showery and rain probably fell at these stations. 

 t A few centimeters core was taken at each depth, 



M 



As the result of lumbering and fires, this formation has been 

 extensively invaded by forms from adjoining open slopes, but typically 



"In this connection see also Jack {op, cU., p. 434, pi. 12); Shaxtz (op. cit., 

 PP- 34i 40); and^RAMALEY {op, cit,, pp. 31 et sen.). 



