426 THE BIOLOGY OF FLOWERING PLANTS 



must be stretched ; with hard-coated seeds freezing of the 

 soil may act through the increased pressure of sharp soil 

 particles. Abrasion by soil particles is likely to be important, 

 and may be intensified for those seeds which pass without 

 damage through the gizzards of birds or even of earthworms. 

 Exact investigation of such points is much required. 



4. Light Effects. — It has been long known that some 

 seeds, such as those of the tobacco, require light for their 

 germination. The mistletoe was thought to require after- 

 ripening which terminated only late in the spring, but 

 Heinricher (191 6) has shown that it will germinate at any 

 time if it is artificially illuminated ; it merely requires a 

 favourable temperature and a large quantity of light. In 

 recent years investigation has shown that a considerable 

 number of seeds can germinate only in light or have their 

 germination promoted by illumination. Examples are 

 Chloris ciliata (a South American grass), Epilohium htrsutum, 

 Veronica longifolia, Lythrum Salicaria, Ranunculus sceleratiis, 

 Rumex crispus, and many Gesneriaceas. A smaller number 

 can germinate only in the dark, e.g. Nemophila insignis, 

 Phacelia tanacetifolia, Veronica Tournefortii, Nigella damas- 

 cena. The cases now known will certainly be added to, and 

 it may be necessary to revise the general impression that, 

 while darkness is the normal condition during germination, 

 m.ost seeds are indifferent to illumination. 



The conditions of germination in such seeds are very 

 complex. We may take first the case of Chloris ciliata 

 investigated by Gassner (1910 I and II, 1911). The fruit, 

 enclosed by the tight-fitting glumes, goes through an after- 

 ripening period of about eight months, during which changes 

 in the embryo probably occur. After this it germinates 

 only in the light, unless the glumes are removed, when it 

 germinates equally well in the dark. If after removal of 

 the glumes it is kept moist and dark at a temperature too 

 low for germination (12° C), then on subsequent trans- 

 ference to a suitable temperature (33° C.) it now requires 

 light, unless the coat at the micropilar end is removed, when 

 it will again germinate in the dark. In the light, after- 



