I. CONCEPT AND HISTORY. 



The practical aspect.— Every plant is a measure of the conditions under which 

 it grows. To this extent it is an index of soil and climate, and consequently an 

 indicator of the behavior of other plants and of animals in the same spot. A 

 vague recognition of the relation between plants and soil must have marked 

 the very beginnings of agriculture. In a general way it has played its part in 

 the colonization of new countries and the spread of cultivation into new areas, 

 but the use of indicator plants in actual practice has remained slight. It is 

 obviously of greatest importance in newly settled regions. However, it is in 

 just these regions that experience is lacking and correlation correspondingly 

 difficult. In fact the pioneer is often misled by his endeavor to transfer the 

 experience gained in his former home to a new and different region. Differ- 

 ences of vegetation and climate, and often of soil as well, make a wholly new 

 complex of relations. As a consequence, the settler is very apt to go astray in 

 reaching conclusions as to the significance of a particular plant. As the coun- 

 try becomes more settled, experience accumulates and makes it increasingly 

 possible to recognize helpful correlations. But this period usually passes too 

 quickly to establish a procedure before the native plants have disappeared, 

 except from roadsides, meadows, and pastures. The manner and degree of 

 utilization of natural meadows and pastures are clearly indicated by the 

 plants in them. Yet it is exceptional that these indicators are recognized and 

 made use of by the farmer. 



The scientific aspect.— On the scientific side, the concept of indicators could 

 hardly be expected to emerge until plant physiology had made a beginning. 

 Looking backward, one discerns something of this idea in the studies of vege- 

 tational changes by King (1685:950), Degner (1729), Buffon (1742:234, 237), 

 and Biberg (1749:6, 27). l It is likewise suggested in the description of stations 

 by Linne* (1751:265) and especially by Hedenberg (1754:73). The basic 

 correlations were made definite by De Luc (1806: Plant Succession, 10) in his 

 studies of succession in peat-bogs and by Schouw (1823: 157, 166) in the clas- 

 sification of plants by habitats. The idea is more or less in evidence in the 

 long series of observations and discussions relating to the chemical theory of 

 the influence of soils. The chief proponents of the chemical theory were Unger 

 (1836), Sendtner (1854), Naegeli (1865), Fliche and Grandeau (1873), Bonnier 

 (1879), Contejean (1881), Hilgard (1888, 1906), and Schimper (1898, 1903). 

 The founder of the physical theory was Thurmann (1849), though his views 

 necessarily placed his results in more or less harmony with the water-content 

 classification of Schouw. The century-old controversy over the chemical 

 theory has centered around the question of the importance of lime in the soil. 

 While the broadening of ecological research has thrown this question more and 

 more into the background, there is still anything but unanimity of opinion 

 concerning it. While it is felt that the problem can be solved only by more 

 comprehensive and tho roughgoing experimentation than it has yet received, 



x Cf. Plant Succession, 1916:8-10; Development and Structure of Vegetation, 1904:12. 



