216 



PLANT STRUCTUEES 



soil, the hypocotyl again rapidly elongates and develops a 

 strong arch, one of whose limbs is anchored, and the other 

 is pulling npon the cotyledons (Fig. 193). This pull finally 

 frees the cotyledons, the hypocotyl straightens, the cotyle- 



FiG. 193. Germination of the garden bean, showing the arch of the hypocotyl above 

 ground, its pull on the seed to extricate the cotyledons and plumule, and the final 

 straightening of the stem and expansion of the young leaves.— After Atkinson. 



dons are spread out to the air and light, and the young 

 sporophyte has become independent (Fig. 194). 



In the grain of corn and other cereals, so often used in 

 the laboratory as typical Monocotyledons, but really excep- 

 tional ones, the embryo escapes easily, as it is placed on 

 one side of the seed near the surface. The hypocotyl and 

 stem split the thin covering, and the much-modified cotyle- 

 don is left within the grain to absorb nourishment. 



In some cases the cotyledons do not escape from the 

 seed, either being distorted with food storage (oak, buck- 

 eye, etc.), or being retained to absorb nourishment from 

 the endosperm (palms, grasses, etc.). In such cases the 

 stem-tip is liberated by the elongation of the petioles of the 



