A certain group of plants of wide geographic range are of particular interest because they are common to low arid regions and high mountain areas but not common at middle elevations. The presence of these species at the extremes in altitudinal distribution would seem at first thought, to be inconsistent with the laws governing zonal distribution. However, a consideration of the environmental and adaptive factors appears to account for the unusual zonal positions.

Under the conditions of a clear and dry atmosphere, radiation takes place more rapidly, causing a corresponding lowering of the temperature at sunset. In this respect it has been found that arid plains and desert regions are identical with the exposed open slopes of the upper mountain areas. Attention is also called to the fact that the plants common to the two regions are usually provided with definite adaptive modifications which have to do with the regulation of their heat, and the conservation of moisture, such as depauperate and depressed forms with special epidermal structures which provide for a slow rate of evaporation to keep them from drying up. They also have highly developed root systems which enable them to reach down for available soil moisture, and at the same time provide an increased amount of absorptive surface which rapidly takes up moisture, providing at the same time a foothold in the loose and shifting material in which the plants often grow. In other words, these structural and physiological characteristics furnish thermal regulation, increased capacity for taking up and retaining moisture, and secure anchorage in the soil.

To sum up, it will be seen that the environmental and other conditions in the two regions are practically identical, excepting in the matter of temperature due to differences in altitude, which last is taken care of by the special adaptations already mentioned, such as nonconducting air-filled hairs.

It is interesting to note that these plants are for the most part of northern origin, having migrated southward with the advance of the continental glaciers. They extended laterally over a wide front from the Sierra Nevada-Cascade Mountains to the Rocky Mountain region, running southward, in some instances to the deserts of the southwest.

Crater Lake National Park and the Lava Beds National Monument furnish an excellent example of the plant distribution here outlined. While all the plants named below are not common to both areas, most of the species are identical. On the northerly and easterly side of the rim of Crater Lake, on the brink of the crater wall, are narrow, precipitous slopes. Usually these are bordered on the upper side by Whitebark Pines (Pinus albicaulis), often dwarfed and hedge-like. In general the soil is deep and made up of pumice sand and gravel. Cloud Cap, on the east rim of Crater Lake, furnishes the best example, possessing the greatest number of species of the group of any similar area within the park.

In the Lava Beds, while a few of the species are widely distributed over the entire area, the greater number are found on the steep exposed slopes of the miniature volcanic cones known as "buttes". These are covered with a good depth of pumice sand with a top layer of cinders, the latter apparently serving as a mulch of moisture conserving material, as does the coarse pumice gravel of the slopes of Crater Lake.

Species common to Crater Lake and Lava Beds

Lava Beds species not occurring at Crater Lake