16 PLANT GROWTH 



The temperature of the soil at planting time plays the 

 most important part in the speed of germination of many 

 seeds because the rate of water absorption and of enzyme 

 activity like all other molecular and chemical actions are 

 regulated by temperature. Because of the difference in the 

 optimum temperature for the enzyme activity, some seeds, 

 such as peas, germinate better at low temperatures than other 

 seeds, such as zinnias. At a temperature below 40° F. it 

 requires six to ten times as long for red clover to germinate 

 as at the optimum temperature of about 60° F. 



Since all the passages through the seed coat are extremely 

 small and are filled with water, the oxygen for respiration 

 must be dissolved in the soil water before it can enter the 

 seed. When the moist soil is packed around the seed, as ex- 

 plained above, the soil moisture comes in contact with the 

 moisture in the passages of the seed coat and the oxygen can 

 enter the seed through these water passageways. The air 

 spaces in the soil supply the oxygen to the water as it is 

 absorbed by the seed. Since the seed needs both oxygen and 

 water, the importance of an optimum amount of each is evi- 

 dent. However, excessive rain causes an oxygen deficiency 

 because as the water content of the soil increases, the soil air 

 decreases. Seeds may fail entirely to germinate in an exces- 

 sively wet soil, also the growth of plants is retarded and their 

 leaves turn yellow with excessive rain. A loose porous soil 

 supplies air most effectively. Chapter 7 describes soil con- 

 ditions more fully. 



Seeds planted nearest to the optimum conditions will 

 germinate quickest. Planting time must be adjusted to the 

 conditions of the soil and temperature, but early planted 

 seeds frequently produce earlier plants than those planted 

 under more favorable conditions even though their rate of 

 germination and growth was slow. The minimum tempera- 

 ture for germination of peas is under 40° F. while for cucum- 

 ber it is above 60° F., hence peas may be planted two or 



