THE PLANT ASSOCIATION. 273- 



phytes. This word comes from hydor, meaning water, and phyton, 

 meaning plant. Those plants growing in soils with an intermediate 

 amount of water are known as mesophytes, that is, literally intermediate 

 plants. Those plants growing in soils that have little water, are known 

 as xerophytes, that is, dry plants. Now it is convenient to use the terms 

 xeromesophytes and hydromesophytes. So under this classification the 

 associations discussed above are as follows: 



A wet meadow is a hydrophytic association. 



An Engelmann spruce forest is a hydromesophytic association. 



A Western larch-Douglas spruce forest is a mesophyitc association. 



A Bull pine-Douglas spruce forest is a xeromesophytic association. 



A prairie is a xerophytic association. 



In the prairie formation there are two places where trees may grow, 

 namely, along streams and on protected hill sides. It is obvious that in 

 the former situations the roots of the trees penetrate to or near to the 

 underground water level, which is not far from the surface. In the latter 

 situation (Plate LII) the fact that trees are protected from drying winds 

 and the soil from the heat of the sun, prevents both trees and soil from 

 drying out rapidly. The plants that grow there can get more water 

 and give off less than they would absorb and transpire if the hill were not 

 present. Thus the protected slope of a hill may have forests in a 

 prairie formation. 



Again, if a hill be high enough to cool sufficiently the moisture-bear- 

 ing winds so as to cause precipitation of moisture, it may get more rain- 

 fall than the lower lying land. This is very probably the reason why the 

 tops of mountains or of high hills in prairie regions have trees and some- 

 times dense forests. This leads to the dicussion of the forest conditions 

 in high mountains. 



The climate towards the top of a mountain is different from that at 

 its base. It is always colder and usually more moist. The moisture 

 conditions are favorable to trees, the low temperature conditions are 

 against tree growth. The slopes exposed to dry winds have less mois- 

 ture for trees than those not so exposed. At the same time the ex- 

 posed slopes receive more heat, rapidly melting the snow, which would 

 otherwise lie longer, and thus shorten the season. As a consequence 

 tree growth is more prevalent on these slopes than on the protected slopes 

 where the snow lies the year round. As one ascends a mountain like 

 MacDougal's peak or Hall's peak, which are in the Swan range bordering 

 the Flathead valley on the east, the species that require the most heat 

 will disappear first. Those that are adapted to a shorter season will be 

 found higher up. The bull pine is one of the first to disappear. Higher 

 up the western larch and silver pine are absent, and on the ridges the 

 Douglas spruce and lodgepole pine are the last of the lowland forms to 

 disappear. In the valleys the Engelmann spruce will be found as high 

 as these or higher. Before the conditions are too severe for the last 

 three named species, the alpine trees come in. In the region under dis- 

 cussion these are the white-bark pine and the alpine fir. (Plate XLVII.) 

 It has already been shown that the latter species is found occasionally 

 in the valley. In the cold canyons it and the Engelmann spruce may 



