424 THE BIOLOGY OF FLOWERING PLANTS 



The same type of dormancy is shown by the apple, the 

 elder, and the lime (Rose, 1919). Eckerson (1913) showed 

 that in the hawthorn the acidity of the embryo increased 

 as after-ripening proceeds, and this must affect metabolism 

 and particularly enzyme action. For Juniperus virginiana^ 

 Pack (1921) found that during after-ripening acidity in- 

 creased, as did soluble sugars, phosphatides, and nitro- 

 genous compounds, and the activity of enzymes. An 

 increase in enzymes and acidity was found by Rose in the 

 lime. The low temperature (0° to 5° C.) which is generally 

 favourable to after-ripening may aid the accumulation of 

 cell-building material by keeping down respiration ; but 

 that respiration is necessary is suggested by the quicker 

 ripening of the hawthorn seeds with the coats removed. 

 The after-ripening process may evidently be regarded as 

 a period of mobilisation of cell-building material. 



3. Seed Coat Effects. — Much more numerous are the 

 cases in which the seed coats and fruit walls inhibit germina- 

 tion, which takes place at once when these are removed. 

 The coats may act by preventing absorption of water, or 

 intake of oxygen, or by mechanical restraint. In Alisma 

 plantago, Crocker and Davis (19 14) found that water 

 absorption takes place readily ; the embryo swells and 

 presses against the restraining walls with an imbibition 

 force of about 100 atmospheres, but cannot rupture them 

 and so is unable to germinate. Treatment with acids and 

 bases induces germination as was shown by Fischer (1907) 

 for Sagittaria. It is likely that these act by altering the 

 mechanical nature of the walls. The achenes of Alisma 

 and of many other water plants such as Sagittaria, Pota- 

 mogeton, Hippuris, Scirpus, and Sparganium, may lie 

 dormant in the mud for many years, and resistance of the 

 fruit wall seems to be the general cause, though oxygen 

 relations may sometimes be involved. The hard endocarps 

 of the bramble druplets delay germination (Rose, 1919). 

 In nature the walls may perhaps be gradually affected by 

 bacterial action or by acids produced in decay. 



The hard-coated seeds of the Leguminosas, Labiatas, 



