i66 



Gardening 



in the seed. A continual supply of water is therefore 

 necessary. 



With the very beginning of growth, the embryo needs 



iSurfcuie of soil 



FIG. 91. The germination of corn, bean, and squash. In the corn (a) the 

 little plant pushes out both stem and root and grows upward, leaving the seed 

 in the ground. In the bean (b) the root grows out and turns downward, and 

 then the part just above the root grows into an arch and pushes upward, dragging 

 the seed with it. After this the seed coat is burst open and the first two seed- 

 ling leaves (cotyledons) are freed. In the squash (c) the seed coat is caught 

 against a small "peg" just above the root and held, while growth forces the 

 cotyledons upward and pulls them out of the old coat. 



an additional and constant supply of oxygen for respira- 

 tion. This comes from the air that is held in the spaces 

 in the soil. But if water completely fills the spaces be- 

 tween the particles of soil in which seeds are planted, the 

 seeds will decay because of lack of air, just as they do if 

 left in a dish with water covering them. Hence seeds 

 germinate best when the soil about them is moist with 

 capillary water but has no free water in it. 



A certain amount of warmth is necessary to start 

 growth in a seed. The amount required is greater for 

 seeds of warm-season vegetables than for those of 

 cool-season plants. In fact, the best temperatures for 



