124 ILLINOIS ACADEMY OF SCIENCE 
1. Autecology. This branch of ecology studies the plant 
as an individual, and is largely physiological in nature. It 
considers (a) the general results of the relation of the plant 
to its environmental factors, as shown in the division of plants 
into great classes according to their reaction to each of the 
leading factors. Chief among these is the moisture relation, 
expressed in the more or less familiar division into hydro- 
phytes or water lovers, xerophytes or dry-climate plants, and 
mesophytes inhabiting an intermediate habitat. A similar 
relation to light and temperature divides plants into sun toler- 
ant and shade-tolerant ; heat-tolerant and cold-tolerant groups. 
The relation to the chemical elements in the soil is not so 
marked as was once thought to be the case, yet we still hear 
such words as “‘calciphiles’’ and calciphobes,”’ and the terms 
probably represent to a certain extent a real situation. (b) 
Special relation to environment is shown by the structure and 
function of the organs. Whatever may be our belief as to the 
method by which variations are produced and fixed in plants, 
it is evident that structures correspond more or less to func- 
tion and are conditioned directly or indirectly by the environ- 
ment. A comparative study of plants in different habitats 
leads us to identify or construct from the imagination certain 
“normal” or original types of organs. We find also modifica- 
tions of these types, which are either temporary, where the 
plant tissues are plastic; or permanent, constituting variations. 
In tracing the correspondence of these changes to environ- 
mental differences, we look for and frequently think we find 
what may be called ecological causes. The best illustration of 
this is shown in a comparison of organs, especially leaves, 
of hydrophytic as compared with xerophytic and mesophytic 
plants. Here there seems to be a very distinct correspondence 
between structure and the markedly different environments 
of these different habitats. 
2. Synecology, which studies plants in the mass is largely 
concerned with distribution of plants, and may be regarded as 
an application of autecology in the grouping of plants within 
greater or smaller areas of the earth’s surface. It may be di- 
vided into (a) ‘“Phytogeography” in which the groupings are 
regional and the result of climatic factors, and (b) “Physio- 
graphic Ecology,” in which the groupings are local, as the 
result of physiography with attendant climatic modifications. 
