270 General Botany 



more rapidly at a high temperature than at a low, with other 

 conditions favorable, seeds will keep longer when the temperature 

 is low. 



External conditions necessary for germination. We have 

 already discussed some of the internal conditions necessary for 

 the germination of seeds. The seed coats must be permeable to 

 water and oxygen, and they must allow complete swelling of the 

 embryo. The embryo must be fully grown and in some cases in 

 an acid condition. 



The first external condition necessary for germination is abun- 

 dant moisture, but there should not be enough moisture to inter- 

 fere with the access of oxygen. Water is needed to bring about 

 the swelling of the cells and tissues ; to dissolve various salts, 

 sugars, and other organic substances in the cells ; and to facihtate 

 chemical changes in the cells. 



Oxygen is needed for respiration. Oxidation liberates energy 

 for chemical changes in the cells. The respiration of germinating 

 seeds goes on at a very high rate ; when compared with that of 

 human beings the rate is several times as great. Human beings 

 give off carbon dioxide equivalent to 2.5 per cent of their dry 

 body weight in 24 hours ; germinating seeds may give off from 5 

 to 20 per cent of their weight of carbon dioxide in a day. 



Seeds are planted as near the surface of the soil as possible to 

 insure an adequate oxygen supply. They are planted below the 

 surface to insure a sufficient supply of water. As the relation of 

 water and oxygen to soil particles varies greatly in different soils, 

 it is evident that to obtain a sufficient amount of both oxygen and 

 water, seeds must be planted deeper in some soils than in others. 

 We may plant them deeper in loose sandy soil, for example, than 

 in tight clay. 



The third important external factor for germination is tempera- 

 ture. Temperatures favorable for germination are usually lower 

 than those for the subsequent development of the plant. But as 

 few seeds germinate much below 50° F., the temperature of the 



