100 



PHYSIOLOGY. 



doing by a hard surface. After the root is 2-^cm long, and the 

 two halves of the seed coats have begun to be pried apart, if we 



look in this rift at the 

 junction of the root 

 and stem, we shall see 

 that one end of the seed 

 coat is caught against 

 heel, or "peg," 

 which has grown out 

 from the stem for this 

 purpose. Now if we 

 examine one which is 

 a little 

 >more ad- 

 vanced, 



we shall see this heel 

 more distinctly, and 

 also that the stem is 

 arching out away from 

 the seed coats. As the 

 stem arches up its back 

 in this way it pries with 

 the cotyledons against 

 the upper seed coat, 

 but the lower seed coat 

 is caught against this heel, and the two are pulled gradually 

 apart. In this way the embryo plant pulls itself out from be- 

 tween the seed coats. In the case of seeds which are planted 

 deeply in the soil we do not see this contrivance unless we dig 

 down into the earth. The stem of the seedling arches through 

 the soil, pulling the cotyledons up at one end. Then it 

 straightens up, the green cotyledons part, and open out their 

 inner faces to the sunlight, as shown in fig. 90. If we dig into 

 the soil we shall see that this same heel is formed on the stem, 

 and that the seed coats are cast off into the soil. 



Fig. 89. 

 Escape of the pumpkin seedling from the seed coats. 



