50 BIOLOGY OP PLANTS. 



2. Ai?\ The oxygen of the air is an active agent in the 

 process of germination. Seeds will not germinate when 

 placed in a vacuum; in an atmosphere of carbonic acid, of 

 nitrogen, hydrogen, or of any other gas, which does not con- 

 tain oxygen. The principal .substance exhaled during the 

 process is carbonic acid. According to Liebig a small quan- 

 tity of acetic acid and ammonia are also formed during the 

 process. These gases form an atmosphere around the seed, 

 unless it comes in contact with water. The volume of oxy- 

 gen consumed is equal to that of the carbonic acid produced. 

 The oxygen of the air either combines directly with the car- 

 bon,* or with the hydrogen of the decomposed water ;t hence 

 this appears to be either a true process of decay, or of com- 

 bustion, and were it not for the vital force, the seed would 

 soon be separated into its original elements. As the oxygen 

 of the air is absolutely essential to germination, some have 

 supposed that the reason why seeds buried too deep, or in a 

 stiff soil, will not germinate, is that they are not reached by it, 

 and have inferred the importance of ascertaining the proper 

 depth for the different kind of seeds in order to facilitate the 

 process. On the same principle they account for the fact, 

 that after deep tillage, plants often make their appearance, 

 which have been cultivated upon the soil several years before. 

 But it should be remembered that seeds thus situated are also 

 deprived of other necessary conditions of which the absence 

 of the oxygen of the atmosphere is probably the least impor- 

 tant. Carbonic acid which is highly useful to the plant is 

 supposed to be injurious to germination by excluding the oxy- 



* " The very first act of life in a seed is to evolve carbonic acid by 

 its carbon combining with oxygen of air, and its second act is to de- 

 compose water." — Dana. 



t " Water is decomposed l)y their vital force ; and its oxygen, com- 

 bining with the carbon, forms carbonic acid." " Seeds have the pow- 

 er of decomposing water wliich causes the commencement of germi- 

 nation. ' ' — Lindiey. 



