LIBERATION OF EMBRYO 435 



responsible for rupture, which then takes place as the 

 embryo resumes active life. Growth of the endosperm, of 

 the cotyledons, of the hypocotyl or of the radicle may be 

 effective. Growth of the endosperm — as opposed to 

 imbibitional swelling — does not seem to be a common 

 occurrence, but Miiller finds that it is responsible for split- 

 ting the testa in Ricinus communis, Pinus Pinea, and other 

 conifers. Much commoner is the growth of the cotyledons ; 

 it is effective in such hard fruits as those of the plums, 

 hazel and walnuts. In these the wall splits along pre- 

 determined lines of mechanically weak tissue. Among the 

 monocotyledons the Cyperacese show this type. 



The growth of the hypocotyl or of the radicle is much 

 the commonest cause of rupture. According to the exact 

 way in which the force is applied Miiller distinguishes a 

 number of sub-types, regarding which we need only note 

 that in some cases the force acts through the endosperm, 

 in others through the cotyledons, or directly. In the grass 

 Coix Lachryma the fruit wall is extremely hard and brittle, 

 but at each end there is a spot occupied by soft fibres through 

 which radicle and shoot make their way. In the coconut, 

 Tradescantia, Potamogeton, Sparganium and others ger- 

 mination takes place by the pushing out of preformed plugs 

 of tissue. Cocos is specially interesting. The endocarp 

 is extremely strong ; at its base are three " eyes," one 

 corresponding to each of the three carpels. Only one seed 

 is, however, present, and only the eye corresponding to this 

 seed provides a plug which can be displaced. As in the 

 hazel and plums already mentioned, so in some other fruits, 

 e.g. Fumaria and AHsma, predetermined, mechanically 

 weaker lines occur, along which splitting takes place. 

 These may be regarded as homologous with the lines of 

 opening of dehiscent fruits. In certain palms (Lepi- 

 docaryeffi), the fruits of which are clad in a scale-like 

 armour, the weak lines form a net-like system. In most 

 seeds there is no special tissue, and rupture is irregular, 

 usually near the point of exit of the radicle. 



Miiller also measured exactly the force involved in the 



