INVASION 221 



In dissemination by offsh(X)ts, the conditions are somewhat similar. Here, 

 also, ecesis comprises the sequence of growth and reproduction, and in- 

 vasion, 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 fol- 

 lowing discussion. 



In accordance with the above, it would be possible to distinguish three 

 groups of terrestrial plants : ( i ) those migrants which germinate and dis- 

 appear, (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 invasion does not occur unless the plant actually takes possession, 

 i. e., reproduces. From the latter statement, it follows that invasion occurs 

 only when a species migrates to a new place, in which it germinates, ma- 

 tures, and reproduces. Maintenance by annual invasion simply, in which 

 the plants of each year disappear completely, can not then be regarded 

 as invasion proper. On the other hand, though such instances are rare, it 

 is not necessary that the invaders produce fruit, provided they are able to 

 maintain themselves, or to increase by propagation. Furthermore, if a plant 

 germinate, grow, and reproduce, it is relatively immaterial whether it per- 

 sist for a few years or for many, since, as we shall see under Succession, 

 the plants of one invasion are displaced by those of the next, the interval 

 between invasions increasing with the stabilization. 



270. Qermination of the seed. The germination of seed or spore 

 is determined by its viability and by the nature of the habitat. Viability 

 depends upon the structural characters of fruit, seed-coat, and endosperm, 

 and to a degree upon the nature of the protoplasm or embryo. The first 

 three affect the last directly, by protecting the embryo against dryness, 

 against injury due to carriage by water, or by deglutition, and probably in 

 some cases against excessive heat or cold. Marloth^ has investigated the 

 structure of seed coats, establishing the following groups, which are sum- 

 marized here somewhat fully because of their bearing upon ecesis: (i) 

 seed coats without protective elements, endosperm absent or rudimentary, 

 Epilohium, Impatiens, Parnassia, Sagittaria, etc.; (2) protective elements 

 lacking or few, endosperm highly developed with thick-walled cells, 



'Uber mechanische Shutzniittel der Samen gegen schadliche Einfliisse von ausson. 

 Engler Bot. Jahrb., 5 :5fl. 1883. 



