Ch. XIV] FORMATIONS AND ASSOCIATIONS 581 



symbiotically linked by bonds of mutual benefit, as in case 

 of mycorhiza with some trees. Possibly some forms may 

 destroy the root excretions of the dominants, to mutual 

 advantage ; and other bonds of such nature are conceivably 

 more prevalent than we yet know. Such forms are sym- 

 bionts. Fourth, the spacing apart of the dominant forms 

 sufficiently for the spread of their own foliage, usually leaves 

 interspaces available for smaller forms which can fit the condi- 

 tions there present. Thus in a forest the dominant trees 

 build a canopy of foliage upheld on the ends of lofty radiate 

 branches. Beneath there is room for tall shrubs, which 

 utilize light that filters through the canopy; and, in like 

 manner, below the shrubs is room for tall herbs, and below 

 them for a ground layer of low herbs, creepers, Mosses, 

 Liverworts, and perhaps Algae. Thus we have forms which 

 are simply interpolated in the interspaces left by the domi- 

 nant kinds, or are interpolants. Fifth, the presence of 

 the independent plants in the formation offers a field for 

 exploitation by dependent plants, — climbers, epiphytes, 

 half-parasites and parasites, while saprophytes profit by 

 their decaying remains ; and these dependents differ with 

 the character of the dominant and interpolated forms. 

 Sixth, special forms may be present as visitants, by 

 chance or accident, as with rare members of the flora. Thus 

 any formation consists of a framework of dominant vegeta- 

 tion forms determined by the habitat, with which others are 

 present as symbionts, as interpolants, as dependents, or as 

 visitants. A formation is thus somewhat of an organism, 

 though of loose aggregation. 



The more intensive study of any formation shows that, 

 while homogeneous in general, it differs in details of consti- 

 tution in different parts. Thus, in our mixed forest form- 

 ations the deciduous trees are more abundant in some 

 places and the evergreens in others ; and the undergrowth 

 differs correspondingly, for each kind is accompanied by 

 somewhat different associates. Such groups, comprising 



