SEED DORMANCY 4^3 



and definite period of ** after- ripening " before they can 

 germinate in suitable conditions ; the case of the hard-coated 

 seeds is only one of many. 



1. Immature Embryos. — In some seeds the embryo is 

 not completely formed when separation from the parent 

 occurs. In Ginkgo, when cultivated in Europs, fertilisation 

 does not take place till after the seed has fallen. In Japan, 

 its native region, fertilisation has taken place and the 

 embryo is sometimes mature, sometimes not — an unusual 

 instance of plasticity. In Ceratozamia and Gnetum the 

 embryo is small and grows after the fall of the seed. The 

 seeds of Ranunculus Ficaria, Anemone nemorosa, Corydalis 

 cava and a few other dicotyledons contain, when they fall, 

 a small, undifferentiated embryo in which development 

 proceeds slowly through the autumn and winter, and is 

 complete just before germination in the spring. Growth 

 of the embryo also takes place in the ivy and probably in 

 some monocotyledons, e.g. Gagea lutea and Paris quadri- 

 folia ; Goebel's account should be consulted. It will be 

 noted that these are woodland plants with a markedly vernal 

 vegetation period, and that this slow after-ripening of the 

 seed leads to germination taking place early in the year 

 following seed formation. 



We may here refer to a class of seeds already con- 

 sidered in another connection, — those of the orchids and of 

 such parasites as the broom-rapes. The seeds are minute 

 and contain an undifferentiated embryo capable of further 

 development, and of germination, only in special conditions 

 — in the presence of the appropriate fungus in the orchids, 

 and of the root of a possible host in the parasites. 



2. Other Embryonic Conditions. — The seeds of Cra- 

 tcegus mollis^ and probably of other hawthorns, contain a 

 fully developed embryo which is, however, incapable of 

 immediate germination. Davis and Rose (19 12) found 

 that, even in the absence of the seed coat, germination did 

 not occur. A slow process of after-ripening takes place, 

 lasting two to three months if the carpel wall is removed, 

 and about one month in the absence of the seed coat. 



