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 established 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, inasmuch 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 
io 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 when 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. 
