PLANT PHYSIOLOGY 235 



not appear to be so important. In plants the inorganic food 

 materials obtained from the soil are essential as building ma- 

 terial, and as physiological activators, but here also they do 

 not contribute directly to the energy of the plant. Because, 

 however, the supply of these materials in the soil is liable to 

 exhaustion and, because the supply of organic food materials as 

 manufactured by the plant itself is practically limitless, these 

 inorganic foods have a much greater relative importance in 

 plants than in animals, and are usually referred to when the 

 term plant food is used. 



Differences between Plants and Animals. — If then animals 

 and plants are alike, as living organisms in the relation of food 

 and energy, wherein lies the essential difference that char- 

 acterizes them as plants and as animals? 



In the discussion which follows we must emphasize the 

 point that we shall speak of general facts and conditions as 

 pertain especially to the higher green plants with which our 

 study is related. In considering all of the varying forms of 

 plant life, from the lowest to the highest, many different condi- 

 tions exist, and plant physiology as a whole must consider them 

 all and correlate the facts into a system, but for our purpose 

 only the main general facts are essential and these we shall 

 obtain from a study of the higher plants. 



Exothermic and Endothermic Reactions. — All chemical 

 reactions are of two kinds in their relation to energy. One 

 kind of reaction liberates or expends energy or is, as we term 

 it, exo-thermic (gives out heat). The other kind absorbs or 

 conserves energy and is termed endo-thermic (takes in heat). 

 The exothermic reaction converts potential energy into kinetic, 

 while the endothermic reaction converts kinetic energy into 

 potential. The exothermic reaction is in general true in cases 

 in which a complex compound is broken down into simpler 

 ones, especially if the reaction involves additional oxygen, that 

 is, if the compound is oxidized. Such a reaction is typified by 

 the oxidation of glucose, as given above, yielding carbon dioxide 

 and water as the products, with the liberation of energy. Re- 



