PROPAGATION BY SEEDS 47 



but are present nevertheless. After a time, however, a point 

 is reached where the life processes cease, and the cells of the 

 embryo lose their power to grow and multiply even when 

 placed under favorable conditions. The length of time seeds 

 retain their viability varies with the kind of seeds as well as 

 with the conditions under which they are matured and stored. 

 Mechanical injury received in threshing, or due to the 

 eating of parts of the seed by insects, may result in inability 

 of the seeds to germinate. 



61. External conditions. The presence of moisture, 

 warmth, and oxygen are the external requirements for the 

 germination of seeds. 



62. Moisture. The first process in germination is the 

 absorption of the proper amount of water. The presence of 

 moisture is necessary because food materials can be trans- 

 ferred from part to part only when in solution. The water 

 renders possible the action of enzymes (ferments) always 

 present in the seed. By action of the enzymes, the stored 

 food material is converted into the forms needed for the 

 growth of the embryo. While the amount of water most 

 suitable for germination varies with different seeds, usually 

 enough should be present to keep the seed saturated, but not 

 enough to exclude the air. 



63. Oxygen is necessary for the growth of the embryo 

 plant. Enough oxygen is present in the soil for the germina- 

 tion of seeds unless the soil is water-soaked. 



64. Heat. A certain amount of warmth is necessary 

 before seeds will germinate. Each kind of seed has an 

 optimum temperature at which it germinates best. It has 

 also a minimum and maximum temperature, the lowest and 

 highest at which it will germinate . The optimum temperature 

 for germination usually is a very few degrees (1 to 4 degrees) 

 higher than the temperature at which the plant makes its 

 best growth. From 60 to 80 degrees F. is satisfactory for the 

 germination of most seeds, but there are exceptions. Seeds 



