COMPETITION AND ECESIS 263 



of the habitat, but also to the changes in them due to compe- 

 tition. Competition is consequently a most important factor in 

 the ecesis of an invader in a formation. It is necessarily absent 

 when an invader enters a new or denuded soil alone, but it appears 

 quickly when a large number of plants invade at the same time. 



Normally, ecesis consists of three essential processes, viz., 

 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 the growing plant 

 or part is usually disseminated, and ecesis consists merely in 

 being able to continue to grow and reproduce. Moreover, ecesis 

 is practically certain on account of the slight differences between 

 aquatic habitats. In dissemination by offshoots, the conditions 

 are somewhat similar, and ecesis consists of growth and repro- 

 duction alone, since the offshoot grows under the same condi- 

 tions as the parent plant. 



With respect to ecesis, migrants may behave in any one of 

 four ways: (1) they may invade an area without germinating; 

 (2) they may germinate and then disappear; (3) they may ger- 

 minate and grow without reproducing; (4) they may reproduce 

 either by flowers or offshoots, or both. Ecesis, and hence in- 

 vasion, occurs only when a migrant enters a new place, in which 

 it germinates, grows, and reproduces. 



279. Germination of the seed. The germination of seed or 

 spore is dependent upon its vitality and upon the nature of the 

 habitat. Vitality, or viability, is determined by the structural 

 characters of the fruit, seed-coat, and endosperm, as well as by 

 the nature of the embryo. The first three promote germination 

 by protecting the embryo against dryness or against injury due 

 to carriage by water or by swallowing. This duty is chiefly 

 discharged by the seed-coat, which usually contains thick-walled 

 protective cells. The seed-coat also helps to insure germination 

 at the proper time, by making the seed more or less resistant to 

 a quantity of moisture and warmth insufficient to support the 

 seedling. The presence of the endosperm or other food supi)ly 

 increases the chances of the seedling. 



The behavior of the seed with respect to germination depends 

 in a large degree upon the nature of the embryo. :\Iany seeds 

 are not viable because fertilization has not taken place and the 

 embryo has not developed. This explains the low germinating 



