224 THE PLANT. X. 



well as the optimum temperature vary with different seeds. 

 The temperature of most rapid germination is usually between 

 25 and 33. 



During germination oxygen is actually absorbed in con- 

 siderable quantity, carbon dioxide being exhaled. 



The process of oxidation produces heat, and a considerable 

 rise of temperature may bo observed when many seeds ger- 

 minate together (e.g., in malting barley). During this slow 

 combustion the fats and carbohydrates are consumed, but the 

 proteid matters remain undiminished in quantity. 



In germination various enzymes present in the seed com- 

 mence to act (diastase, which has the power of converting 

 starch into maltose and dextrose, is a typical one), whereby 

 the reserve materials in the seed are converted into soluble 

 substances capable of transportation in the sap. The embryo 

 increases rapidly in size, sending upwards a plumule, which 

 will eventually produce the stem, and downwards a radicle, 

 destined to form the root. The direction of growth of the 

 plumule and radicle is determined by gravitation, for if seeds 

 be germinated while subjected to centrifugal force (say on a 

 rotating wheel) the plumule grows towards and the radicle away 

 from the centre of rotation. As soon as the plumule reaches 

 the surface of the soil^and becomes exposed to light, chloro- 

 phyll is produced in it and assimilation commences. The 

 radicle, too, develops root-hairs and becomes capable of taking 

 in plant food from the water of the soil. 



Before describing the chemical changes which occur in 

 plants it may render the matter clearer if a few terms which 

 are used in physiology be explained. 



Metabolism refers to all chemical changes which occur in or 

 are produced by living matter. These are further subdivided 

 into 



(I) Anabolism or constructive metabolism, including all pro- 

 cesses in which complex compounds are built up from 

 simpler ones by the aid of living organisms. The final 

 stage of all anabolic processes is the formation of proto- 

 plasm. In the case of plants the most characteristic 

 anabolic process is the synthesis of carbohydrates from 



