THE ENVIRONMENT 



327 



from their lowland relatives in ability to withstand 

 great extremes of temperature. Most invertebrates 

 can withstand high mountain minimal temperatures 

 without reaching the critical point at which their 

 body fluids freeze. For example, some land snails can 

 survive — 184°F. if they are warmed gradually. How- 

 ever, some invertebrate adaptations can be related to 

 high mountains. More of the alpine species are black, 

 thereby absorbing the heat of the sun. Many of the 

 flying insects have instincts causing them to fly close 

 to the ground, thereby avoiding the upper zone of 

 strong winds. Some insects are even wingless, in 

 contrast to their lowland relatives. 



Perhaps the most startling information about 

 alpine life was the discovery of jumping spiders plus 

 glacier fleas and related springtails at 22,000 feet and 

 above. All of these animals are wingless and dark- 

 bodied, the dark body every day absorbing the 

 heat necessary to thaw the organisms out of the 

 glacial ice. The jumping spiders feed upon the glacier 

 fleas and springtails, the fleas and springtails feed 

 upon fungi and decaying plants at lower elevations — 

 but far above this level, well into glaciers where no 

 plants are known, the animals feed upon windblown 

 materials, mostly small insects and plant pollen. 



SOIL 



Soils normally support a vast number of organ- 

 isms. The plants and animals usually have adapta- 

 tions associated with certain features of their soil 

 environment. For example, burrowing animals are 

 frequently limited to specific soil conditions, espe- 

 cially textural, in direct relation to the animals' 

 physical capabilities for burrowing. Plant correla- 

 tions with their soil habitat are often more funda- 

 mental. Among the soil factors of greater importance 

 to plants are soil texture, organic matter, moisture, 

 pH, and salinity. 



SOIL TEXTURE 



Various generalizations are possible as to the rela- 

 tionships between plants and soil texture (Table 

 17.4). (1) Fine-textured silt and clay soils offer the 

 greatest resistance to plant root penetration, and the 

 coarser textured soils offer least resistance. (2) Water 

 penetrates coarse soil readily and there is little runoff, 

 but finer-textured soils retard water penetration and 



TABLE 17.4 ASSOCIATIONS BETWEEN SOIL TEXTURE 

 AND PLANTS' 



^The conditions generally more beneficial to plants are starred. 



erode more readily owing to the greater runoff. In 

 nature this may mean that two habitats having much 

 the same precipitation have different plants. For ex- 

 ample, a sandy site may have a forest which requires 

 more available moisture, and a clayey area may have 

 grassland. (3) Water moves more slowly through 

 finer-textured soils. Because water moves rapidly 

 through sandy soils, sandy soil plants often must have 

 deeper roots. However, in sandy soils water may rise 

 between the grains by capillary attraction, eliminat- 

 ing the need for deep-rooted plants. (4) Fine-textured 

 soils tend to hold more water. This feature is an ad- 

 vantage of fine soils over coarse ones in places subject 

 to drought periods. The actual holding of water is 

 related directly to the surface area of soil particles, 

 the combined area of spaces between particles, and 

 the amount of water imbibed by individual particles. 

 Since each of these physical features is best developed 

 in fine soils, fine soils have greater water retention 

 than coarse soils. (5) Fine soils usually are more 

 fertile, because they are physically better suited for 

 storing nutrients. (6) Coarser soils because of greater 

 porosity have faster evaporation, better aeration, and 

 closer agreement with atmospheric temperature 

 changes than do finer soils. (7) Few soils are simply 

 fine or coarse; rather, most are aggregates of finer 

 particles among larger particles and display both the 

 good and bad features of their particle sizes. 



ORGANIC MATTER 



Plants rely upon soil minerals as a source of nutri- 

 ents; but usable inorganic sources of minerals are 

 limited. Most minerals required by plants are de- 



