XII PREFACE. 



Both communities and species may be studied with respect to the 

 phylogenetic relationship of the species concerned and their places 

 in the natural system of classification. The phylogenetic study of 

 individual species has formed the sharply defined field of taxonomy, 

 but the study of communities from the standpoint of the phylogenetic 

 relationship of their component individuals has long been such a 

 prominent part of plant geography that it has remained as a large 

 element in ecological activities. The investigation of the causes 

 which determine the distribution of plants and plant communities is 

 essentially a physiological task, in which it is necessary for us to 

 regard the plant as a functioning organism and to give little attention, 

 for the time being, to the fact that it has a descent-kinship with other 

 plants. We must keep the plant in mind as an aggregation of coordi- 

 nated physiological processes, continually controlled by a complex 

 of environmental conditions. It is only by a sharp separation of tlje 

 phylogenetic and the physiological considerations of the plant that 

 we can hope to investigate with success the relation of plants to their 

 environmental controls. The physiologist has thus far been mainly 

 interested in the individual processes of the plant as affected by the 

 environmental conditions acting singly. The ecologist is interested 

 in the collective activities of the plant, as controlled by the entire set 

 of environmental conditions and as measured by the dispersal, estab- 

 lishment, growth, reproduction, and survival of the plant in a state of 

 nature. He is further interested in the assemblages of plants which 

 occupy the same natural situations or habitats, which appear to be 

 subjected to closely similar sets of environmental conditions and 

 appear to meet these environmental complexes by closely similar or 

 dissimilar types of physiological behavior. In short, the physiologist 

 has mainly investigated the absolute value of conditions by the pre- 

 arranged and controlled methods of experiment, while the ecologist 

 investigates the relative value of these conditions as they cooperate 

 to influence the plant, endeavoring to determine which of them are 

 effective in determining habitat and distribution, and what intensities 

 are of critical importance in this connection. He is also especially 

 interested in the combination of the many different kinds of environ- 

 mental conditions, as these form the infinite variety of environmental 

 complexes furnished by nature. 



While plant geography is an old science, with a large literature, and 

 while the newer science of ecological plant distribution already pos- 

 sesses numerous monographs that present types of vegetation, plant 

 associations, etc., as related, in a general way, to environmental con- 

 ditions, yet these studies have usually been primarily descriptive of 

 the vegetation itself, and but little has yet been accomplished in the 

 way of corresponding descriptions of the environmental conditions 

 that are observed to be concomitant with the various forms of vegeta- 



