628 



GERMINATION AND DORMANCY 



tures the seed coat. This is followed by the emergence of the primary root 

 which develops from the lower end of the hypocotyl and is the first structure 

 of the embryo to make contact with the external environment. The primary 

 root grows downward in the soil producing lateral roots and root hairs. The 

 hypocotyl then elongates rapidly, pulling the cotyledons upward out of the 

 soil into the air where they separate into an approximately horizontal posi- 

 tion on either side of the plumule. The plumule then begins active growth 

 giving rise to the stem and foliage leaves of the seedling. Since the bean 

 is a seed without an endosperm the food used during germination is largely 

 derived from the accumulations in the thick cotyledons. 



Fig. 138. Stages in the germination of a seed of lima bean (Phaseolus lunatus). 



2. Seeds in Which the Cotyledons Do Not Emerge. — The seed of the pea 

 is structurally very similar to that of the bean but its germination behavior is 

 very different. Elongation of the hypocotyl does not occur and the cotyledons 

 remain in the seed. The primary root elongates early in the process of germina- 

 tion much as in the bean. The plumule is elevated through the soil by 

 rapid elongation of the epicotyl, which is the stem region between the coty- 

 ledons and the first true leaves — in other words the first internode. 



This type of germination is also exhibited by oak acorns (Fig. 139). 



Many monocots also show this type of germination behavior. In the 

 germination of the corn grain, for example, the primary root develops from 



