54 INVASION 



ized, Caryophyllaceae, Crassulaceae, Fumariaceae, Saodfragaceae; 

 (2) parenchyma thick-walled, several-layered, Aesculus, Gas- 

 tanea, Fagus ; (3) parenchyma cells with the inner or radial 

 walls thickened, Campanula, Erythraea, Gentiana; (4) epider- 

 mal cells cup-shaped, thick-walled, Cruciferae, Bibes, Vacd- 

 nium; (5) parenchyma with thickened, cellulose walls, 

 Geranium, Viburnum; (6) a single row of stone-cells, Ldbiatae; 

 (7) tissue of stone-cells, Eippuris, Naias, Potamogeton; (8) 

 elongate stone-cells, Goniferae, Gupuliferae, Euphorbia, Linum, 

 Malva, Viola; (9) short columnar thick- walled branched cells, 

 Gucurbitaceae, Datura, Hypericum; (10) prosenchyma with 

 cellulose walls, Glematis; (11) prosenchyma with lignified 

 walls, Fraxinus, Ehamnus, Ranunculus. The seed-coats have 

 a certain influence in determining germination at the proper 

 time, in as much as they make it difiBcult for the seed to ger- 

 minate under the stimulus of a quantity of warmth and 

 moisture insufficient to support the seedling. The effect of 

 the endosperm, as well as that of other food-supply in the 

 seed, upon germination and the establishment of the seed, 

 ling is obvious. 



The behavior of seed or spore with respect to germina- 

 tion depends in a large degree upon the character of the 

 protoplasm or embryo, though in just what way is at pres- 

 ent a matter of conjecture. It is evident that many seeds 

 are not viable because fertilisation has not been effected, 

 and in consequence no embryo has developed. This is the 

 usual explanation of the low germinating power of the seed 

 of some species, especially polyspermatous ones. But even 

 in viable seeds the behavior is always more or less irregular. 

 The seeds of some species will grow immediately after ripen- 

 ing, while others germinate only after a resting period of 

 uncertain duration. The same is true of spores. Even in 

 the case of seeds from the same parent, under apparently 

 similar conditions, while the majority will germinate the 

 first year, some will lie dormant for one or more years. 

 The precise reason why many seeds and spores germinate 

 more readily after being frozen is equally obscure. The 

 period of time for which disseminules may remain viable is 



