132 GROWTH OF THE SEED 



see that the upper end of the hypocotyl reacts to gravity like a 

 stem and consequently grows away from the earth. This growth 

 is attended with a curving and constant nutation of the hypo- 

 cotyl which enables it to work its way through the ground to 

 better advantage than would be the case if it were pushed pas- 

 sively up. Darwin compares this work of the seedling in emerg- 

 ing from the soil to the struggles of a man who is prostrate and 

 attempting to arise with a heavy load on his back. The ability 

 of the hypocotyl to overcome obstacles in its way and crowd 

 through small openings is due to the power generated in the 

 enlarging cells. In this way a force is often created that may 

 exceed 80 pounds to the square inch, over five times the atmos- 

 pheric pressure. In many seeds only the region above the coty- 

 ledons elongates, leaving the cotyledons in the ground, as in the 

 pea, acorns, walnuts, etc. The elongating region is called the 

 epicotyl. This behavior is doubtless associated with the storage 

 of food. In some seeds the cotyledons serve only as storage or- 

 gans and in such cases the position of advantage is in the soil 

 where they are better protected and in contact with moisture 

 which will assist in the solution and diffusion of the foods. In 

 many cases the cotyledons perform a dual function, first as storage 

 organs and later expanding and developing into the first green 

 leaves of the plant, as in the squash and many of our common 

 plants. In any of these cases it is evident that the young plant 

 requires a certain amount of food to enable it to get established 

 in the soil. It is not until considerable growth has taken place 

 that it develops its first green leaves and so begins to be self 

 supporting. In the case of the bean and in many other seeds and 

 fruits this need is amply met by the abundance of food that is 

 stored in the fleshy cotyledons (Fig. 84, C). These organs re- 

 main attached to the plant even after it is well established, and it 

 is only after the food has been withdrawn that the shriveled 

 cotyledons drop off. As the young plant becomes established 

 the plumule unfolds like a bud and develops the stem and leaves 

 and finally the flowers and fruit with which you are familiar. 



The castor bean shows quite a variation from the structure 

 of the bean. The hilum and micropyle are more or less con- 



