PHYSIOGRAPHIC FACTORS 



175 



in wet lowlands are invariably shallow-rooted, for here aeration is 

 poorest because the soil is periodically or continuously saturated 

 and the only available oxygen may then be in solution. These 

 shallow-rooted species will usually grow well in uplands, but, if 

 the naturally deep-rooted species are moved to lowlands, they do 

 not do well or may actually die. Thus aeration may determine the 

 rate of growth, an element of importance in forest stands, and may 

 be the factor controlling the type of vegetation. 



Soil Aeration and Plant Adaptations.-Well-aerated soils may 

 have an air capacity of 60 to 70 percent by volume, a condition 

 determined primarily by structure and scarcely affected by tex- 

 ture. The amount of air varies, of course, with the water content 

 of the soil, for air is forced from the spaces in the soil that become 

 occupied by water. 



Thus continuously saturated soil is poorly aerated, and the mud 

 under a pond probably has the poorest aeration of any plant habi- 



FlG. 84. Some types of lacunar tissue found in stems of emergent and 

 other aquatic vascular plants. (A) Cortex of water milfoil (Myriophyllum). 

 (B) Ground parenchyma throughout stem of a rush (J uncus). (C) Same for 

 a sedge (Cyperus). 



