GERMINATION. 367" 



undergo a chemical change, and that an elevation of 

 temperature is produced. The vital power is dormant 

 it sleeps in the seed until the proper conditions are 

 produced. It has been proved, that the powers of 

 maintaining life in the seed are very great ; excessive 

 cold, sufficiently intense to freeze mercury, will not kill 

 seed,, and they resist a comparatively high temperature. 

 It is probable that heat only destroys seeds by drying 

 them too completely. The temperature at which 

 seeds germinate is exceedingly varied, those belonging 

 to our own clime will germinate when the thermometer 

 rises above 40. F., but the seeds of tropical plants 

 demand that a temperature of from 70 to 84, or even 

 to 90, be steadily applied to them. In some cases it 

 has been found that even boiling the seeds has been 

 advantageous to the future process of germination in 

 the soil. But let us take the seed of some ordinary 

 plant, and trace its progress. 



An apparently dead grain is placed in the soil. If 

 the temperature is a few degrees above the freezing point, 

 and the soil holds a due quantity of water, the integument 

 of the seed imbibes moisture and swells ; the tissue is 

 softened, and the first, effort of vital force begins. The 

 seed has now the power of decomposing water, the 

 oxygen combines with some of the carbon of the seed, 

 and is expelled as carbonic acid. Saussure's experiments 

 prove this. The air above the soil in which a horse- 

 bean was placed to germinate, gave, before the ex- 

 periment, nitrogen 210*26, and oxygen 56*29, and after 

 germination, nitrogen 209'41, oxygen 44*38, and car- 

 bonic acid 11*27. This part of the process is but little 

 removed from the merely chemical changes which we 

 have already considered. We find the starch of the 

 seed changed into gum and sugar, which affords nutritive 

 food for the developing embryo. The seed now 

 lengthens downwards by the radicle, and upwards by 



