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 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, matures, 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 persist 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. Germination 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[33] has investigated the structure of seed coats, establishing the following groups, which are summarized here somewhat fully because of their bearing upon ecesis: (1) seed coats without protective elements, endosperm absent or rudimentary, _Epilobium_, _Impatiens_, _Parnassia_, _Sagittaria_, etc.; (2) protective elements lacking or few, endosperm highly developed with thick-walled cells, _Liliaceae_, _Primulaceae_, _Rubiaceae_, etc.; (3) protective cells present in the seed coats, endosperm little or none, _Boraginaceae_, _Crassulaceae_, _Cruciferae_, _Labiatae_, _Papilionaceae_, etc.; (4) protective elements present, _Asclepias_, _Campanula_, _Gentiana_, _Silene_, _Saxifraga_, etc.; (5) protective cells present, endosperm thick-walled, _Euonymus_, _Helianthemum_, _Ribes_. The protective cells are of various kinds: (1) epidermal cells strongly cuticularized, _Caryophyllaceae_, _Crassulaceae_, _Fumariaceae_, _Saxifragaceae_; (2) parenchyma thick-walled, several-layered, _Aesculus_, _Castanea_, _Fagus_; (3) parenchyma cells with the inner or radial walls thickened, _Campanula_, _Erythraea_, _Gentiana_; (4) epidermal cells cup-shaped, thick-walled,