24 PLANT HABITS AND HABITATS IN THE 



amount of moisture in the soil reported on by Shreve could be con- 

 siderably below that available for -plants. Accordingly the arid fore- 

 summer in southern Arizona constitutes very largely a resting season for 

 plants. Since this season comprises about 3 months without rain, it 

 can be concluded that as long, or longer, rainless periods as occur in the 

 dry parts of Australia may operate to bring about a condition of ex- 

 treme soil dryness and that under such circumstances only in favorable 

 situations, or in favorable soils, or in species which reach to deeply 

 placed soil moisture, or which have a water-balance, can vegetational 

 activities be carried on. 



Studies on the relation between the moisture-content of the soil and 

 the condition of permanent wilting of plants indicate that all species 

 wilt at approximately the same moisture-content in the same soil, other 

 conditions being equal. Thus, contrary to previously accepted belief, 

 plants native to dry regions are unable "to reduce the water-content 

 of the soil to a lower point than is reached by other plants at the time of 

 wilting" (Briggs and Shantz, 1912:235). 



Although possibly the largest percentage of water escapes from the 

 soil through evaporation from its surface, a very considerable amount is 

 lost by reason of transpiration from the plant shoot. This goes on 

 until the limit of water loss is reached only by the establishment of an 

 equilibrium between air and soil, and the final result is the same as if 

 the air and soil were in direct contact (Briggs and Shantz, 1912: 20). 

 Not only does the upper soil layer lose moisture through the plant cover, 

 but the deeper layers as well become dry by the same means. Thus 

 it has been determined (Alway, McDole, and Trumbull, 1919:185) 

 that the moisture of the subsoils may be greatly reduced through the 

 action of deeply rooted plants that is, whose roots are 5 meters or less 

 in length. Where such deep-rooted perennials are wanting, the sub- 

 soil remains moist. 



AERATION OF THE SOIL. 



The aeration of the soil is an environmental factor of plants of much 

 consequence, although it is measured with difficulty and there appears 

 to be no way of expressing it concretely or exactly. Data regarding 

 this phase of environment, therefore, are largely wanting, but it is 

 known in a general way that the soils of the dry regions are, as a whole, 

 well aerated. This follows from the known conditions directly affect- 

 ing air-movements in soils. Among these the following may be men- 

 tioned: Size of the soil grains; compactness of the soil; amount of 

 moisture in the soil; winds and differences in barometric pressure; 

 temperature of the air and of the soil itself; the plant cover. 



The composition of the soil is also an important feature in its aera- 

 tion. In the upper soil layers the atmosphere of the soil usually has 

 about the same composition (except possibly as to moisture-content) 

 as the atmosphere immediately above it. Under conditions of re- 



