436 THE BIOLOGY OF FLOWERING PLANTS 



rupture of the fruit wall of the hazel-nut and of the seed coat 

 of Pinus Pinea and Ricinus communis ; in the first the growing 

 cotyledons exert a force of 3*3 atmospheres ; in the others 

 the force developed by the growing endosperm amounts to 

 37 and 3 "I atmospheres respectively. These values are 

 smaller than might have been expected. Miiller has shown, 

 however, that they suffice to rupture the walls and coats, 

 though only in the soaked condition ; in the dry state the 

 resistance is much higher. In Corylus the resistance of 

 the fruit wall falls from about eight atmospheres when dry 

 to about three when wet. In Pinus Pinea a fall to a third 

 of the value when dry took place on wetting. The same is 

 true of thin-walled seeds ; thus the testa of the bean is six 

 times as resistant when dry as when wet. The force re- 

 quired to expel the plug of Cocos campestris is less by 40 

 per cent, after soaking. 



§ 6. Germination— Emergence of the Seedling 



With the radicle free from the seed coat germination 

 proceeds, but the rest of the embryo, or some part of it, may 

 remain enclosed longer or permanently. In the germination 

 of the pea the epicotyl frees itself later than the radicle, 

 but the cotyledons remain within the seed coat until both 

 wither or rot away. When the cotyledons remain in the 

 soil we speak of germination as hypogeal, when they emerge 

 it is epigeal. Both types may occur in a single genus ; thus 

 Phaseolus muliiflorus, the scarlet runner, is hypogeal, Phase- 

 olus •vulgaris y the French bean, is epigeal. In the lupin and 

 sunflower the cotyledons carry the coats above ground as 

 a cap, which is got rid of sooner or later as the cotyledons 

 enlarge and spread apart. This happens in many cases, 

 and the young seedlings with their bonnets have a rather 

 quaint appearance. But frequently the carrying up of 

 the seed coat is a matter of chance. If the seed is well 

 buried the resistance of the soil may retain the coat, while 

 if the seed lies on the surface the cotyledons may remain 



