THE PHYSICAL ENVIRONMENT OF ORGANISMS 557 



some of the smaller vertebrates hidden in crevices or beneath protecting 

 rocks and leaves when they are not in search of food. Nearly all the 

 crayfishes and lobsters furnish very good examples of this type of reaction. 



An interesting and characteristic society of small animals that live 

 at the surface of quiet waters (marsh pools and small ponds) is partially 

 to be accounted for by the positive thigmotropism of many of its members 

 to the air-water surface film. This film is sufficiently stable to serve as a 

 "ceiling," where small aquatic animals may cling or move about, and to 

 serve as a floor for a rather diverse population of small arthropods. 



Gravity. Many plants exhibit very clear-cut reactions to gravity. 

 The embryonic root at germination always exhibits a very strong positive 

 geotropism and " insists" on growing downward, even though the young 

 plant is turned upside down after growth has begun. The embryonic 

 leaf shoot, on the other hand, shows a definitely negative geotropism and 

 grows upward, even if this necessitates overcoming considerable inter- 

 ference. The bending of the root or stem results from unequal rates of 

 growth on its opposite sides. Such differential growth is caused by local- 

 ization, under the influence of gravity or light, of definite growth- 

 stimulating substances — hormones called auxins. In general, geotropism 

 is more characteristic of plants than of animals, but a number of animals 

 show a definite geotropism. 



The Soil as an Environmental Complex 



Soil may be defined as the layer of mixed mineral and organic material, 

 penetrated by plant roots, that covers at least 95 per cent of the land 

 surface. It lies at the contact of the atmosphere with the lithosphere, 

 and is subject to repeated wetting and partial drying, to leaching in its 

 upper layers and cementation in its middle and lower layers, to changes 

 of temperature, and to the influences of the plants and animals that live 

 upon and within the soil. 



It would appear logical to consider the soil as one of the media in or 

 upon which organisms live, but, unlike air or water, the soil is so complex 

 and varies so widely from place to place that in its relationships to the 

 organism it combines the roles of medium and modifying factors. We 

 have already seen that the soil is the storehouse for a large list of neces- 

 sities for organic life and that its characteristic biota of bacteria, fungi, 

 and animal life has a very important part to play in the energy cycle. 

 Just as temperature, light, and humidity may vary widely in intensity 

 and quantity and thus produce widely different environmental conditions, 

 the variations in soil are likewise of great importance in governing both 

 the geographic and local distribution of organisms. 



In order to appreciate the complexity of the soil and its great variability 

 it is necessary to know something of the parent materials and the processes 



