436 PLANTS AND THEIR RELATION TO MAN 



FIG. 302. Harrowing loosens the soil. 



must be a goodly supply of fresh air for the seeds. If a field 

 in which seeds have just been planted is trampled on so that 



the ground is packed 

 down, the seeds do not 

 sprout. This is because 

 air cannot penetrate be- 

 tween the particles of 

 packed soil, and without 

 air seeds cannot respond 

 to heat and moisture. 

 The farmer plows and 

 harrows his fields before 

 planting seed in order to 

 loosen the soil and to 

 make new channels through which the air can penetrate (Fig. 

 302). The earthworm is helpful to the farmer because in wind- 

 ing its way through the soil, it leaves openings through which 

 the air can reach the seeds. 



A large part of the moisture which the seed needs for germina- 

 tion soaks through the seed coat, but some makes its way to the 

 embryo through a small pmlike opening 

 near the scar on the seed (Fig. 303). The 

 soil must be packed close around the seed 

 in order that the seed ccat andopeningmay 

 take moisture from it, but the soil must 

 be packed loosely enough to admit the air 

 necessary for growth and germination. 



Food for the seedling. Food is neces- 

 sary to start the seed in its growth, and 

 the food which the embryo plant needs to start its growth is 

 stored in the seed. In the bean the thick swollen cotyledons, 

 or halves of the seed, contain food (Fig. 304), which supplies 

 the seedling until it is ready to forage for itself. The food 



FIG. 303. A bean, show- 

 ing opening a. 





