434 THE BIOLOGY OF FLOWERING PLANTS 



plum, or the coconut, which remain enclosed in the strong 

 endocarp of the fruit, or of the sunflower and other 

 Compositae, and the hazel-nut, where the whole fruit wall 

 is hard and persistent. The imbibition forces of the colloids 

 of the sweUing seed may amount to hundreds of atmospheres, 

 but do not usually come into play. In indehiscent fruits 

 the ripe seed frequently does not occupy the whole of the 

 available space, so that the swelling embryo has room to 

 expand without exerting pressure on the fruit wall — as in 

 the hazel, or plum, or sunflower. Where the swelling 

 embryo does press on the fruit wall, as in the achenes of 

 Alisma plantago, even the great force developed may be 

 powerless to effect rupture, as Crocker and Davis (19 14) 

 have shown, and this takes place after changes in the fruit 

 wall have altered its mechanical properties. In the case of 

 many seeds and fruits which swell greatly on soaking in water 

 the envelopes also swell and expand, so that no splitting takes 

 place. This is seen, for example, in the thin-coated pea or 

 bean, in the hard-coated broom, and in the fruits of the 

 cereals. 



An exact investigation by G. Miiller (191 4) deals with the 

 mode of liberation of the embryo in a large number of cases. 

 He finds that rupture of the envelopes by imbibitional 

 swelling of embryo or endosperm is quite uncommon ; it 

 takes place, for example, in Ipomcea purpurea and in some 

 legumes. After a few hours in water the embryo of Ipomcea 

 swells to thrice its dry volume, and the testa is split by a 

 net-Uke system of cracks. The rarity of this mode of 

 liberation may seem strange, since the forces of imbibition 

 are the most powerful at the disposal of the plant. Miiller 

 sees a biological advantage in this, since damage to the 

 embryo might result if swelling caused rupture of the coats, 

 for absorption of water need not be accompanied by other 

 conditions favourable to the growth of the young plant. 

 We may note that the possibility of seed coat dormancy would 

 be largely lost, and that a dormant embryo would be exposed 

 to bacterial and fungal attack by the bursting of the coat. 



In the great majority of plants the forces of growth arc 



