ECESIS 53 



invasion does not occur unless the plant actually takes pos- 

 session, i. e., reproduces. Prom the latter statement, it 

 follows that invasion occurs only when a species migrates to 

 a new place, in which it germinates, matures and reproduces ; 

 maintainance by annual invasion simply, in which the plants 

 of each year disappear completely, cannot then be regarded 

 as invasion proper. On the other hand, though such 

 instances are rare, it is not necessary that the invaders pro- 

 duce fruit, provided they are able to maintain themselves, or 

 to increase by propagation. Furthermore, if a plant germi- 

 nate.grow and reproduce.it is lelatively 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 increas- 

 ing with the stabilisation. 



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 endos- 

 perm, and to a degree upon the nature of the protoplasm or 

 embryo. The first three affect the last directly, by protect- 

 ing the embryo against dryness, against injury due to carri- 

 age by water or by deglutition and, probably in some cases, 

 against excessive heat or cold. Hegelmeier (1873), Lohde 

 (J874), Bachmann (I880»and particularly Marloth (1883:225), 

 have investigated the structure of seed-coats. The latter 

 established 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, Sag- 

 ittaria, etc. ; (2) protective elements lacking or few, endo- 

 sperm highly developed with thick-walled cells, Liliaceae, 

 Primulaoeae, Bubiaceae, etc. ; (3) protective cells present in 

 the seed-coats, endosperm little or none, Boraginaceae, Cras- 

 sulaceae, Cruciferae, Labiatae, Papilionaceae, etc. ; (4) protec- 

 tive elements present, Asclepias, Campanula, Gentiana, Silene, 

 Saxifraga, etc. (5) protective cells present, endosperm thick- 

 walled, Euonymus, Helianthemum, Bibes. The protective cells 

 are of various kinds : (1) epidermal cells strongly cuticular- 



