368 Vegetable Physiology. 



warmth, moisture, and the presence of oxygen. The process 

 is also favored by darkness. The influence of each of these 

 agents will be readily understood. No vital action can go on 

 without a certain amount of heat ; and where this is not pro- 

 duced within the being, it must be derived from without. The 

 germination of a seed is therefore as much dependent upon 

 warmth, as the hatching of an egg, though the amount required 

 is not nearly as great. Moisture is also required, for the con- 

 version into a fluid state, of the dry nutriment which has been 

 previously stored up in the seed ; and no change can com- 

 mence until that is supplied. The presence of oxygen is ne- 

 cessary, because the conversion of starch into sugar requires 

 that some of the carbon of the former should be set free. For 

 this purpose the carbon must be combined with oxygen so as 

 to form carbonic acid. This process is favored by darkness, 



because light has a tendency to produce the contrary effect 



the fixation of the carbon. 



It is interesting to observe how all these conditions are sup- 

 plied, in the ordinary course of nature, by the soil in which 

 the seed is dropped. If it be sown during the spring or sum- 

 mer, it speedily begins to germinate ; but if in the autumn, it 

 remains almost unchanged, until the winter has passed, and 

 the returning warmth of the earth and air arouses it into acti- 

 vity. It is seldom that the soil is so completely destitute of 

 moisture, for any long time together as not to be able to excite 

 seeds to germinate ; but their sprouting is well known to be 

 favored by damp weather ; and if seeds remain undeveloped 

 on account of having been placed in the ground during a 

 drought, they are very rapidly brought forward by a shower. 

 A porous soil is favorable to germination, on account of the free 

 admission of air as well as moisture, which it affords to the 

 seed. 



