32 HARPER 



3. Terrestrial (edaphic). 



Water. 



Amount present in soil, or depth if covering the surface. 

 Average. 

 Seasonal variations. 



Substances held in suspension or solution. 



Temperature. 



Movements (especially whether flowing or stationary). 

 Soil and subsoil. 



Presence or absence of Lafayette. 



Thickness of Columbia. 



Alluvium, if any 

 Humus. 



4. Organic (biotic). 



Plants. 



Equal (associates). 



Inferior (vines, epiphytes, parasites, etc.). 

 Superior (furnishing nourishment, support, or shade). 

 Animals (man excepted). 

 Beneficial, by 



Pollination or dissemination. 



Food for carnivorous plants. 



Influence on soil and humus. 

 Injurious or destructive. 



5. Frequency of fire. 



B. Past history. Changes in 



1. Environment. 



Climate. 



Topography and soil, by 



Elevation and subsidence. 



Erosion and sedimentation. 

 Accumulation of humus. 



2. Vegetation itself, by 



Evolution, mutation, hybridization, etc. 



Rate of variation, and time elapsed. 

 Extinction. 

 Migration. 



Agencies and routes. 



Barriers. 



Time and space. 



C. Properties of the species. 



Adaptations to environment. • 



Growth and reproduction. 



Dissemination. 



Geographical distribution, past and present. 



Some of these factors are essentially the same throughout the 

 region, as is assumed to be the case for instance with climate, 

 which has just been discussed. Those factors which vary in com- 

 paratively short distances give the flora whatever diversity it has. 



