PLANTING THE SEED 25 



below the freezing point, and most seeds germinate very 

 slowly, if at all, below 40 Fahrenheit. The "optimum" 

 temperature, or that at which seeds germinate best, varies 

 with different kinds of seeds, but the range is comparatively 

 narrow for any one kind. The optimum temperature for 

 most of the small grains is around 80 Fahrenheit, though 

 germination begins at about 40. Cotton and corn grow 

 best around 95, and cotton will not germinate at all much 

 below 55. Air is necessary for germination; for oxygen, 

 which is an important constituent of air, is needed for certain 

 chemical changes which take place in the plant food stored 

 in the seed. Moisture is also needed, for these changes 

 take place only when water is present; it also furnishes a 

 medium by which the food supply is carried to all parts of 

 the young plant. Plant food from outside sources is not 

 necessary for germination, nor is light. These are required 

 for continued grow r th, but germination will take place with- 

 out them. (See laboratory exercises at the end of this 

 chapter.) When planted in the soil, the radicle naturally 

 goes down, while the plumule pushes up to the light, no 

 matter in what position the seed is planted. 



22. Planting the Seed. To apply these facts in a practi- 

 cal way, we can readily see that it, is useless to plant most 

 seeds till the soil and the air are warm, though such plants 

 as oats and wheat grow best at fairly cool temperatures. 

 For this reason, they can be sown much earlier than cotton 

 or corn can be planted. The soil should be fine and mellow, 

 and the seed should not be covered too deeply, otherwise the 

 necessary supply of air will be shut off and the supply of 

 plant food in the seed will not be sufficient to enable the 

 young plant to reach the surface. A soil that is cloddy or 

 crusted is unfavorable for germination, as it is more diffi- 

 cult for the tender shoots to force their way through it. 



