222 THE FORMATION 



JMiaceae, Priniulaceae, Ruhiaceae, etc.; (3) protective cells present in the 

 seed coats, endosperm little or none, Boraginaceac, Crassulaccae, Cruciferae, 

 Labiatae, Papilionaccae, etc, ; (4) protective elements present, Asclcpias, 

 Campanula, Gentiana, Silcnc, Saxifraga, etc.; (5) protective cells present, 

 endosperm thick-walled, Euonymns, Ilcliantliemiun, Ribcs. The protective 

 cells are of various kinds: (i) epidermal cells strongly ciiticularized, 

 Caryophyllaccae, Crassulaccae, Fumariaceac, Saxifragaccae ; (2) paren- 

 chyma thick-walled, several-layered, Aesculus, Castanca, Fagiis; (3) 

 parenchyma cells with the inner or radial walls thickened, Campanula, 

 Erytliraca, Gentiana; (4) epidermal cells cup-shaped, thick-walled, Cruci- 

 ferae, Ribes, Vaccinium; (5) parenchyma with thickened, cellulose walls. 

 Geranium, Viburnum; (6) a single row of stone-cells, Labiatae; (7) tissue 

 of stone- cells, Hippuris, Naias, Potamogeton; (8) elongate stone-cells, 

 Coniferae, Cuptiliferae, Euphorbia, Linum, Malva, Viola; (9) short, colum- 

 nar, thick-walled branched cells, Cucurbitaccae, Datura, Hypericum; (10) 

 prosenchyma with cellulose walls, Clematis; (11) prosenchyma with lignified 

 Vv'alls, Fraxinus, Rhamnus, Ranunculus. The seed-coats have a certain in- 

 fluence in determining germination at the proper time, inasmuch as they 

 make it difficult for the seed to germinate 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 seedling is obvious. 



The behavior of seed or spore with respect to germination depends in 

 a large degree upon the character of the protoplasm or embryo, though in 

 just what way is at present a matter of conjecture. It is evident that many 

 seeds are not viable because fertilization has not been effected, and in con- 

 sequence no embryo has developed. This is the usual explanation of the 

 low germinating power of the seeds of some species, especially polysperma- 

 tous ones. But even in viable, seeds the behavior is always more or less 

 irregular. The seeds of some species will grow immediately after ripening, 

 while others germinate only after a resting period of uncertain duration. 

 The sanie is true of spores. Even in the case of seeds from the same 

 parent, under apparently similar conditions, while the majority will germi- 

 nate 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 disscminules 

 may remain viable is extremely diverse, though, as would be expected, it 

 is much longer as a rule for seeds than for spores. The greater vitality of 

 seeds in the case of ruderal plants suggests that this diversity may be due 

 simply to variation in the vigor of the embryos. It would seem that under 

 proper conditions seeds may retain their viability for an indefinite period. 



