6 Plant Physiology 



through the soil. This division of the factors is not wholly 

 satisfactory, as will be seen. Agronomy is commonly sub- 

 divided nowadays into " soils ' and " crops "; but these 

 do not exclude physiology, they make it, in its broadest 

 relations, more nearly indispensable. 



Conditions acting above the soil are mainly sunshine, 

 heat, precipitation, humidity, evaporation, and air move- 

 ment and composition ; while through the soil there is af- 

 forded fixity, mineral nutrients, and water, as well as heat 

 and air. There are, however, many other factors acting 

 above or below the soil which may affect vegetation directly 

 or indirectly, including the bacterial flora of the soil, and 

 injurious substances in the soil solution; fungous diseases, 

 insect pests, and higher animals ; the constant factor grav- 

 ity ; and many special stimuli. 



This complexity of environmental factors renders an 

 interpretation of the effects of any one factor in terms 

 of plant behavior peculiarly difficult. Progress in field 

 study, or experimental ecology, is nevertheless being made, 

 and much is due to the greater perfection in instrumenta- 

 tion as applied to the habitat. 



In nature a species of plant thrives best where the re- 

 quirements of the particular form are most completely 

 met. In the case of the cultivated plant, however, while 

 the natural factors condition production, these alone are, 

 of course, insufficient to determine whether or not a par- 

 ticular crop should be grown in a particular locality, for 

 there are a host of economic considerations which must 

 have weight. Market, transportation, labor, machinery, 

 and many other factors must be taken into account. 



From the point of view of the natural factors there are 



