220 THE FORMATION 



ECESIS 



269. Concept. By the term ecesis is designated the series of phenomena 

 exhibited by an invading disseminule from the time it enters a new forma- 

 tion until it becomes thoroughly estabhshed there. In a word, ecesis is 

 the adjustment of a plant to a new habitat. It comprises the whole process 

 covered more or less incompletely by acclimatization, naturalization, accom- 

 modation, etc. It is the decisive factor in invasion, inasmuch as migra- 

 tion is entirely ineffective without it, and is of great value in indicating the 

 presence and direction of migration in a great number of species where 

 the disseminule is too minute to be detected or too little specialized to be 

 recognizable. 



The relation of migration to ecesis is a most intimate one : the latter 

 depends in a large measure upon the time, direction, rapidity, distance, and 

 amount of migration. In addition, there is an essential alternation between 

 the two, masmuch as migration is followed by ecesis, and the latter then 

 establishes a new center from which further migration is possible, and so on. 

 The time of year in which fruits mature and distributive agents act has a 

 marked influence upon the establishment of a species. Disseminules designed 

 to pass through a resting period are often brought into conditions where 

 they germinate at once, and in which they perish because of unfavorable 

 physical factors, or because competing species are too far advanced. On 

 the other hand, spores and propagules designed for immediate germination 

 may be scattered abroad at a time v/hen conditions make growth impossible. 

 The direction of movement is decisive in that the seed or spore is carried 

 into a habitat sufficiently like that of the parent to secure establishment, 

 or into one so dissimilar that germination is impossible, or at least is not 

 followed by growth and reproduction. The rapidity and distance of migra- 

 tion have little influence, except upon the less resistant disseminules, conidia, 

 gemmae, etc. Finally, the amount of migration, i. e., the number of mi- 

 grants, is of the very greatest importance, affecting directly the chances 

 that vigorous disseminules will be carried into places where ecesis is possible. 



Normally, ecesis consists of three essential processes, germination, growth, 

 and reproduction. This is the rule among terrestrial plants, in which mi- 

 gration regularly takes place by means of a resting part. In free aquatic 

 forms, however, the growing plant or part is usually disseminated, and 

 ecesis consists merely in being able to continue growth and to insure re- 

 production. Here establishment is practically certain, on account of the 

 slight differences in aquatic habitats, excepting of course the extremes, fresh 

 water and salt water. The ease indeed with which migration and ecesis are 

 effected in the water often makes it impossible to speak properly of invasion 

 in this connection, since aquatics are to such a large extent cosmopolitan. 



\ 



