62 INVASION 



SO dissimilar that germination is impossible, or at least is 

 not followed by growth and reproduction. The rapidity and 

 distance of migration have little influence, except upon the 

 less resistant disseminules, conidia, gemmae, etc. Finally, 

 the amount of migration, i. e., the number of migrants, 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 migration 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 reproduction. Here establishment is practi- 

 cally certain, on account of the slight differences in aquatic 

 habitats, excepting of course the extremes, fresh water and 

 salt water. The ease indeed wibh 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. In dissemination 

 by offshoots, the conditions are somewhat similar. Here 

 also, ecesis comprises the sequence of growth and repro- 

 duction, and invasion, in the sense of passing from one 

 habitat to another, is of rare occurrence, as the offshoot 

 grows regularly under the same conditions as the parent 

 plant. The adjustment of growing plants and parts is so 

 slight, and their establishment so certain on account of their 

 inability to migrate into very remote or different habitats, 

 that they may be ignored in the following discussion. 



In accordance with the above, it would be possible to 

 distinguish three groups of terrestrial plants, (1) those 

 migrants which germinate and disappear, (2) those which 

 germinate and grow but never reproduce, (3), those which 

 reproduce, either by propagation or generation, or both. 

 Such a classification has little value, however, since the 

 same species may behave in all three fashions, depending 

 upon the habitat to which it has migrated, and since 



