14 PHaiNOMENA OF GERMINATION. 



called into life, each finding itself in the situation most suitable 

 to its existence ; that is to say, the root in the earth, the stem 

 in the air. 



The conditions required to produce germination are, exposure 

 to moisture, and a certain quantity of heat ; in addition, it is 

 necessary that a communication with the atmosphere should he 

 provided, if germination is to be maintained in a healthy state. 

 A seed, when fuUy ripe, contains a larger proportion of carbon 

 than any other living part, and so long as it is thus charged 

 with carbon, it is unable to grow. The only means it possesses 

 of ridding itself of this principle, essential to its preservation, 

 but forming an impediment to its development as a new plant, 

 is by converting the carbon into carbonic acid, for which 

 purpose a supply of oxygen is necessary. It cannot obtain 

 oxygen in sufficient quantity from the air, for it is cut off 

 from free communication with the air by various means, either 

 natural, as being inclosed in a thick layer of pulp, or in a hard 

 shell or stone ; or artificial, as being buried to a considerable 

 depth below the surface of the soil. It is from the water 

 absorbed in germination that the seed procures the requisite 

 supply of oxygen ; fixing hydrogen, the other element of water, 

 in its tissue: and thus it is enabled to form carbonic acid, 

 which it parts with by its respiratory organs, until the propor- 

 tion of fixed carbon is lowered to the amount suited to its 

 growth into a plant. 



It has been objected that the evidence adduced in support of this 

 explanation is not conclusive ; and that there is nothing to show that 

 the hydrogen of decomposed water enters into new combinations or is 

 fixed ia tissue. But since no hydrogen is evolved during germination, 

 it must necessarily be fixed or recombined after water has been decom- 

 posed. That this last phenomenon occurs is proved by the experiments 

 of Edwards and Colin, as given in the Comptes rendus (vii. 922), and 

 quoted in Lindley's Introduction to Botany (4th edit., II. 261 and 272). 



But the formation and respiration of carbonic acid takes 

 place most freely, though not exclusively, in darkness; if 

 exposed to light, the seed again parts with some of its oxygen, 

 and again fixes its carbon by the decomposition of its carbonic 

 acid. 



