COMPETITION AND ECESIS 265 



281. Adjustment to the habitat. Growth and reproduction 

 in the new place or habitat depend upon the habitat form, the 

 plasticity, and the vegetation form of the plant. Even though 

 it may germinate, a typical shade plant, such as Impatiens, will 

 not thrive in an open meadow, nor will characteristic sun plants, 

 such as most grasses, grow in deep shade. Xerophytes do not 

 adapt themselves to hydrophytic habitats, nor hydrophytes to 

 xerophytic conditions. However, many mesophytes possess to a 

 certain degree the ability to adjust themselves to somewhat xero- 

 phytic or hydrophytic situations, while plants of open wood- 

 land often invade either forest or meadow. This capability for 

 adjustment, i.e., plasticity, is greatest in intermediate species, 

 those that grow in habitats not too wet or too dry, too sunny or 

 too shaded. It is least in forms highly specialized with respect 

 to water content or shade. Hence the rule is that ecesis is con- 

 trolled largely by the degree ,of similarity between the old and 

 the new habitat, except in the case of plastic species, which possess 

 a wider range of adjustment. 



The vegetation form of the invading species is often of the 

 greatest importance in determining whether it will be established. 

 It is characterized by modifications Avhich were probably pro- 

 duced in the original home by competition, anfl are of primary 

 value in securing and maintaining a foothold. These are prac- 

 tically all devices for enabling the plant to persist, such as root, 

 rootstock, bulb, tuber, woody stem, etc. They find their greatest 

 development among trees and shrubs, and their least among 

 annual herbs. 



Experiment 67. Influence of habitat form upon ecesis. Make a culture 

 for the study of ecesis by sowing in a meter plot a given numl)er of 

 seeds of species representing amphibious plants, sun plants, shade jilants, 

 and xerophytes, and growing them under mesophytic conditions. Note 

 the number of seeds that germinate, and the number of individuals of 

 each species that succeed in producing flowers. 



282. Barriers. Any feature of the topography or vegetation 

 that restricts or prevents invasion is a barrier. Such features are 

 usually permanent, and produce permanent barriers, though a 

 barrier is often temporary, existing for a few years only, or for a 

 single season. Barriers are distinguished as complete or incom- 

 plete with respect to the thoroughness with which they limit 



